Encyclopedia of Animal Science

965 Pages·2005·72.83 MB·English

by  Wilson G. Pond

D
EncyclopediaofAnimalScience 1
DOI:10.1081/EEAS120019427
Copyright 2005byMarcelDekker,Inc.Allrightsreserved.
University ofIllinois,Urbana, Illinois,U.S.A.
Stanley E. Curtis
INTRODUCTION threatoractualshiftinsomeinternalorexternalfeature,it
reactstopreemptorcounteractthatchange.Itattemptsto
Sound animal husbandry depends on application of keep an internal steady state, and thereby to survive and
scientific knowledge of many aspects of the biology of thrive. The essence of an animal’s homeokinetic mech-
the animals we keep. Environmental aspects of animal anismsissimilartothatofahome’s simplethermostat:a
care are based on application of principles of animal negative-feedback control loop.
ecology in design, operation, troubleshooting, and cor-
recting deficiencies. They are crucial to both economical
Coping
animal production and responsible animal stewardship.
An environmental adaptation refers to any behavioral,
functional, immune, or structural trait that favors an
animal’s fitness its ability to survive and reproduce
ADAPTATION
under given (especially adverse) conditions. When an
animal successfully keeps or regains control of its bodily
Any environment has factors that threaten to overwhelm
integrity and psychic stability, it is said to have coped.
its inhabitants. Animals are driven to adapt to their
Agivenstimuluscomplexprovokesdifferentresponses
environments, and thereby remain fit. Adaptation is an
by different animals, and even by the same animal from
animal’s adjustment to its environment, especially a
time to time. Tactics vary. Its response depends on the
nonideal one, so its life and species can continue.
individual’s inherent adaptability, accumulated life expe-
riences, current adaptation status, and current ability to
Realistic Expectations muster extraordinary responses.
Animalssometimesfailtoadapt;theyexperiencestresses
of various kinds. So they may feel well, fair, or ill
(described later). We should expect an animal to STRESS
experience well-being mostly, fair-being sometimes, ill-
being once in a while. When an animal shows signs of Failure to Adapt
failing to adapt, correcting the problem may not be easy.
Stress occurs when the stimulation an animal is ex-
periencing goes beyond that individual’s ability to adapt.
Animal Responses Environmental stress may ensue when the environ-
ment changes, adaptation status changes, or an animal
An animal’s environment consists of a complex of is moved to another environment. When an animal
elements, each of which varies over time, across space, has coped, its response is an adaptive response. But
in intensity. Most combine in additive fashion as they there always are limits to adaptability. When attempts
affect an animal. to adapt fail, the response is a stress response, the stimu-
lus a stressor.
Internal steady state Failure to adapt stress has negative consequences
for animal state of being. Understanding untoward
An animal normally maintains steady states over time in consequences of such breakdowns for bodily integrity is
the various aspects of its internal environment. This relatively clear-cut. But psychic disturbance or collapse
mechanism homeokinesis is the general basis of is often not even recognized. It is now believed that
environmental adaptation. When an animal perceives a humans can survive stress only totheextentwe can cope
Adaptation and Stress: Animal State of Being
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2 AdaptationandStress:AnimalStateofBeing
psychologically.Likewise,IanJ.H.Duncan thinksthat Measuring Impacts
animal state of being has to do with animal feelings.
Impacts of environmental impingements are estimated
by measuring their effects on the animal. The same
environment that would quickly chill to death a newborn
COPING piglet might be well-tolerated by the sow. Differences in
thermal adaptabilities of the two put the same environ-
The numerous possible strategies and tactics for counter- ment in the piglet’s cold zone, the sow’s neutral zone.
acting stimuli an animal usually has at its disposal imbue
flexibility and power to the animal’s adaptive responses
Tolerance Limits, Collapse, and Death
when it faces an adverse environment. But when an
animal responds to environmental stimuli, it is not
An animal ordinarily is confronted by more than one
necessarilyunderstressordistress.Respondingtostimuli
stimulus at a time. Stimuli also impinge sequentially.
is a normal biological feat routinely carried out by every
Animals in practical settings generally need to cope with
normal,unstressedcreaturethatlives.Typicalscenariosof
multiple stimuli.
environmental stimuli and animal responses run a wide
A range of tolerance sets limits for an environmental
gamut. Modified versions of nine schemes created by
factors within which an animal can readily cope, thrive,
Donald M. Broom and Kenneth G. Johnson follow:
reproduce, survive i.e., experience wellness. Outside
thisrangearetheupperandlowerrangesofresistance.If
1. In the face of stimuli, internal steady state is main-
ananimalresideslongenoughoutsideitstolerancerange,
tainedwithordinarybasalresponses.Stateofbeingis
it eventually will die due to environmental stress.
very well.
2. Complete adaptation achieved with minor extraordi-
nary response. Stimuli provoke adaptation. Fitness Kinds of Stress Response
and performance may be briefly compromised, but
wellness promptly returns. There are four kinds of stress response. Some reduce an
3. Sometimes, animal response to stimuli over time is animal’s state of being; others enhance it. Understress
neitherextraordinarynoradequate.Forsolongasthe occurs in simple environments that lack certain features
impingementcontinues,fitnessandperformancemay (social companions, play items) (stimulus underload).
be reduced minor stress and fairness ensue but Sometimes animals give behavioral signs of understress
after that, wellness returns. (lethargy; exaggerated, repetitive activity apparently
4. Stimuli elicit someminorextraordinary response, but devoid of purpose (stereotypy); some other disturbed
over time this is inadequate for complete adaptation. behavior). Eustress (good stress): situations of extraordi-
Both fitness and performance decrease awhile (fair- nary responses, but which the animal finds tolerable or
ness),afterwhichwellnessreturns.Stressispresentat even enjoyable. Overstress: environmental situations that
scheme 4 and above. provoke minor stress responses. Distress (bad stress):
5. Ananimal’sextraordinaryresponseoveralongperiod circumstances that provoke major stress responses.
achievesonlyincompleteadaptation.Althoughfitness Judging from signs of negative emotions (anxiety, fear,
remains relatively high, performance is reduced. The frustration, pain), distress causes an animal to suffer, but
animal experiences overall fair-being. to what extent is not yet known.
6. To completely adapt, an animal sometimes must
mount an extreme response. During adaptation and
recovery periods, fitness and performance decline.
The animal is only fair. STATE OF BEING
7. Despite some extraordinary response to stimuli,
complete adaptation is not achieved long term. Fit- Ananimal’sstateofbeingisdetermined byanyresponse
ness and performance decline; the animal becomes the environment requires and the extent to which the
ill. animal is coping. When readily adapting, the animal is
8. Insomecases,anextremeresponsedoesnotresultin well.Whenhavingsomedifficulty,itisfair.Whenfrankly
complete adaptation even long term reducing the unabletocope,itisill.Inreality,environmentsthatmake
ill animal’s fitness and performance. animals ill are not uncommon. But it is our moral
9. An environmental stimulus may be so enormous and responsibility to minimize such occasions and correct
swiftthattheanimal succumbsbeforeitcanrespond. them to the extent possible.
[3]
[4]
[5]
[5]
[6]
AdaptationandStress:AnimalStateofBeing 3
Scientific Assessment mal’stoppriorities.Otherperformanceprocessesmaynot
be critical to an individual’s survival or reproduction, so
Our understanding of an animal’s state of being depends they are least protected and least spared.
on generally accepted observations, scientific laws and When an animal responds to any stimulus, its main-
theories, and unique individual experiences. In 1983, tenance needs invariably increase. Resource expenditures
MarianStampDawkinsandIanJ.H.Duncanbelievedthat insupportofmaintenanceprocessesincreaseprogressive-
the terms ‘‘well-being’’ and ‘‘suffering’’ would be very ly along with stress intensity, so the animal’s potential
difficult to define. That remains the case two decades performance capabilities progressively decrease.
later. Until more is known, it is unlikely that kept
animals will enjoy more of the objectively defined well-
How Animal Responses Affect Performance
being for which we all should hope. Following are some
questions to be asked in assessing animal state of
Environmental stimuli provoke an animal to respond,
being. Is the animal
whichinturncaninfluenceperformanceprocessesinfive
ways. Responses:
. Having its actual needs met, achieving internal
integrity and psychic stability, coping, adapting?
1. Alter internal functions. As an unintentional conse-
. Showing frank signs of sickness, injury, trauma,
quence, certain stress hormones secreted as part of
emotional disturbance?
long-term adaptive or stress responses can reduce a
. As free of suffering as possible, experiencing mostly
foal’s growth rate.
neutral and positive emotional states?
2. Divert nutrients from other maintenance processes
. Tosomeextentabletocontrolitsenvironment,predict
and performance. A nursling piglet that increases
it, live harmoniously in it?
metabolic rate simply to keep its body warm in a
. Performing growing,reproducing,lactating,compet-
chilly environment will have fewer nutrients left for
ing, working at a high level?
disease resistance and growth.
. Showing signs of imminent illness or being in a vul-
3. Directlyreduceanimalproductivity.Thermoregulatory
nerable state?
responses to hot environments sometimes include
reducing internal heat production. Eggs laid by heat-
stressed hens weigh less than normal, due partly to
Animal Needs
decreased feed intake, partly to a homeokinetic re-
ductionineggsynthesis(whichgivesoffheat).
When an animal actually needs something it does not
4. Impair disease resistance. As a consequence, e.g.,
have, it is experiencing a deficiency. At any moment, an
individual feedlot cattle under social stress due to
animal has specific needs based on its heredity; life
aggressive group mates are more likely to become
experiences; bodily, psychic, and environmental condi-
infected and diseased.
tions.Givenitsneedsatagivenpoint,then,thebiological,
5. Increase variation in animal performance. Individual
chemical, and physical elements of its environment
animals differ in responses to stimuli and therefore
determine whether those needs are being fulfilled.
in performance even when residing in the same ad-
verse environment. Stress increases individual varia-
tion in performance.
Functional Priorities Under Stress
A performing animal is one that is producing some
product, progeny, or work or performing some activity Other Considerations
useful to humans. The rate of performance of a
constitutionally fit animal usually is the best single Other environmental aspects of animal care include the
indicator of that animal’s state of being. When its concepts of optimal stimulation, enrichment, predictabil-
performancewanes,theanimalprobablyisnotaswellisit ity, controllability, frustration, and helplessness.
could be.
When bodily resources become limiting as often
happens during stress some processes must be down- CONCLUSION
playedsoothersmorevitalatthemomentcanascend.The
goals of individual survival (maintenance) and species Foundations of success in environmental aspects of
perpetuation (reproduction) in that order are an ani- animal care are the fundamental principles of animal
4 AdaptationandStress:AnimalStateofBeing
3. Duncan, I.J.H.; Dawkins, M.S. The Problem of Assessing
‘‘Well Being’’ and ‘‘Suffering’’ in Farm Animals. In
Indicators Relevant to Farm Animal Welfare; Smidt, D.,
Ed.;MartinusNijhoff Publishers: Boston,1983.
4. CAST. The Wellbeing of Agricultural Animals; Curtis,
S.E., Ed.; Council on Agricultural Science and Technol
ogy: Ames, IA, 1997.
5. Curtis, S.E.; Widowski, T.M.; Johnson, R.W.; Dahl, G.E.;
1. Duncan, I.J.H. Feelings of Animals. In Encyclopedia of McFarlane, J.M. Environmental Aspects of Animal Care;
Animal Rights and Animal Welfare; Bekoff, M., Meaney, Blackwell PublishingProfessional: Ames,IA, 2005.
C.A., Eds.;Greenwood Press: Westport, CT, 1998. 6. The Biology of Animal Stress: Basic Principles and
2. Broom, D.M.; Johnson, K.G. Stress and Animal Welfare; Implications for Animal Welfare; Moberg, G.P., Mench,
Kluwer Academic Publishing:Amsterdam, 1993. J.A., Eds.; CABI Publishers: New York,2000.
ecology and their application. Every situation is complex
and unique. There are no general recipes in these mat-
ters.Thefundamentalprincipleshavebeensetforthhere.
REFERENCES
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[1]
EncyclopediaofAnimalScience 5
DOI:10.1081/EEAS120034100
Copyright 2005byMarcelDekker,Inc.Allrightsreserved.
University ofIllinois,Urbana, Illinois,U.S.A.
Janeen L. Salak-Johnson
INTRODUCTION most circumstances the biological cost (in terms of
biological function) is minimal for acute stressors, but
Duringthedailyroutinesofanimals,theanimalrespondsto during prolonged stress the cost is significant, thus
numerouschallengeswithavarietyofresponses,including leading to a prepathological or pathological state. The
structural and behavioral changes in the brain and body, stress response elicited by a stressor protects the animal
whichenablebothbehavioralandphysiologicalstabilityto and restores homeostasis, thus enhancing the probability
bemaintained. Insomeincidences, adaptivephysiological of survival.
changes are not sufficientto achieve the animal’srequire- The stress response initiated by a stressor results in
ments and in these situations, defense mechanisms are the release of neurotransmitters and hormones that serve
initiated, which are collectively referred to as stress as the central nervous system’s (CNS) messengers to
responses.Stressisatermthatisgenerallyassociatedwith other parts of the body. The CNS obtains information
negativeconsequences,butstressisnotalwaysbad.Often, from the external environment and signals to the or-
organisms seek stress and relish the euphoric feeling and ganism that a particular danger or threat to homeostasis
reward associated with stressful experiences (e.g., skiing, has been perceived. The perception of the threat is
copulation).Thetermstressisfullofambiguities;thus,no mostly related to prior experience and the physiological
clearuniversaldefinitionhasemerged.Forthisdiscussion, state of the animal (Fig. 1). Once the threat has been
‘‘stress’’ is defined as a perceived threat to homeostasis, perceived, adaptive responses are initiated by evoking
which elicits behavioral and physiological responses. The well-orchestrated defenses that include behavioral and
stress response consists of a complex array of behavioral physiological adjustments. Neuroendocrine changes are
and physiological adaptive changes that are initiated as initiated to meet energy requirements for behavioral
a means of restoring homeostasis. Exposure to adverse responses and to maintain homeostasis. It is the final
stimuli results in a well-orchestrated series of responses stage of the stress response that determines whether the
thatcantypicallycausealterationsinautonomic,neuroen- animal is simply experiencing a brief disruption in ho-
docrine, or immune function along with complex changes meostasis with no significant consequences or experi-
in behavior. These homeostatic mechanisms enable the encing extreme difficulty, which may lead to the de-
organismtomaintainbehavioralandphysiologicalstability velopment of disease. Oftentimes, the consequences of
despite fluctuating environmental conditions. the stress response are adaptive in nature. However, if
the animal reaches a state in which the intensity and
duration of the stressor is severe and uncontrollable,
compromising health and reproduction, this condition
HISTORICAL—CONCEPT OF STRESS may lead to development of a prepathological state
or pathology.
Life exists by maintaining a complex of dynamic
equilibrium or homeostasis that is constantly challenged
by internal and external adverse stimuli; often these
stressful conditions are too demanding for the animal to NEUROENDOCRINE RESPONSES
adapt. However, animals have evolved mechanisms that
enable them to adapt to the numerous stressors in their The neuroendocrine responses to stressors are important
lives. An animal can initiate several types of biological adaptationandcopingmechanismsthatoccurinresponse
responses to alleviate stress. These responses often result to a threatening stimulus. The adaptive changes initiated
in shifts or alterations in biological resources that are by stressors involve activation of the hypothalamic-
normally used for other basal functions. Thus, under pituitary-adrenal (HPA) axis. The hypothalamus and the
Adaptation and Stress: Neuroendocrine, Physiological,
and Behavioral Responses
[3]
6 AdaptationandStress:Neuroendocrine,Physiological,andBehavioralResponses
Fig.1 ThisdiagramdepictstheactivationoftheHPAaxisinresponsetostress.TheresponseisperceivedandorganizedintheCNS,
whichinturnactivateseithertheendocrinepathwayorfightorflightresponsesothattheanimalcanreturntohomeostasis.Thetypeof
response(s)theanimal initiates isdependent uponvarious modifiers.
brainstem are pivotal regions of the brain that control the and behavioral adaptive processes, whereas high levels
animal’sresponsetostress.Oncethethreattohomeostasis of cortisol have detrimental effects on various regula-
is perceived, the HPA axis is activated and the hormones tory processes such as immune and neuroendocrine sys-
corticotropin releasing hormone (CRH) and vasopressin tems. The behavioral and physiological effects of CRH
(VP) are released fromthe neuronsof the paraventricular and cortisol are often independent of one another; how-
nuclei (Fig. 1). CRH stimulates the pituitary gland to ever, cortisol can influence CRH neurons by inhibiting
secrete adrenocorticotropin hormone (ACTH) and other and affecting the responsiveness of CRH neurons. Cor-
peptides (i.e., b-endorphin). VP plays a role in sustaining tisol can lead to increases in CRH production and ex-
HPA responsiveness and, along with CRH, has a syn- pression in various regions of the brain. In fact, behav-
ergisticimpactonACTHsecretion.ElevatedACTHstim- ioral responses are influenced by cortisol, facilitating
ulates the adrenal cortex to increase synthesis and pro- CRH expression.
duction of glucocorticoid hormones and regulates the
secretion of glucocorticoids.
The glucocorticoids influence homeostasis and the
biological response to stress. The glucocorticoids are PHYSIOLOGICAL RESPONSES
essential for regulating basal activity of the HPA axis
and terminating the stress response. Glucocorticoids Numerous physiological changes are associated with the
terminate the stress response through an inhibitory feed- stress response that enables the animal to adapt to
back loop at the pituitary and hypothalamus (Fig. 1). aversive stimuli. Short-term activation of the HPA axis
Further responsiveness within the HPA is dependent results in changes in metabolic responses such as rapid
upon this negative feedback, which is influenced by mobilization of energy stores for initiation of the fight-
HPA facilitation. In addition, stress activates the or-flight response. In the long run, suppression and
secretion of the catecholamines, which influence the changes in other physiological responses such as ana-
HPA axis, and mediates many changes associated with bolic processes, energy stores, and the immune system
the stress response. have negative consequences. Stress results in mobili-
zation of energy stores to maintain normal brain and
Cortisol and CRH Expression muscle function while increasing glucose utilization,
which are essential to maintaining physiological stabil-
Cortisol is secreted under diverse conditions that impact ity. Cardiovascular output and respiration are enhanced
both physiology and behavior. Short-term cortisol during stress to mobilize glucose and oxygen for the
release is protective and facilitates normal physiological tissues. The gastrointestinal tract during acute stress is
[5]
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[5]
[4]
[6]
[3]
[4]
AdaptationandStress:Neuroendocrine,Physiological,andBehavioralResponses 7
inhibited. Many of these changes are associated with stress for a prolonged period of time or is in a state in
stressful events that prepare the animal for fight or which behavioral adjustments are no longer adequate
flight. These precise physiological changes are geared to that other physiological processes are affected, leading
alter the internal milieu in order toincreasesurvivability, to a prepathological state or development of pathology.
but if activated frequently and for too long, the results It is this point in which behavioral adjustments are no
can be detrimental. longer adequate to return to homeostasis.
The immune response and processes involving cel- The central state of the brain orchestrates the be-
lular growth and reproduction are temporarily inhibited havioral responses in anticipation of and in adaptation to
during stress to allow the animal to utilize biological environmental events. Behavioral responses to stress
resources for other purposes (such as flight). Long-term involve neuronal systems in which peptides function as
stress can cause disruptions in reproductive physiology neurotransmitters. It has been suggested that CRH coor-
and sexual behavior. Stress modulates the immune sys- dinatesbehavioralresponsestostresssuchasfeedintake,
tem. Acute or short-term stress may suppress, enhance, anxiety-like behaviors, arousal, learning, and memory
or have no effect on the immune system. Chronic or just to name a few. CRH is a critical mediator of stress-
long-term stress can suppress the immune system, thus related behaviors and its influence on behavior is
making it more difficult for the animal to fight disease dependent on the baseline arousal state of the animal.
effectively. Glucocorticoids and other components may In nonstressed animals under low levels of arousal, CRH
contribute to stress-induced immunosuppression, but can is behaviorally activating while under stressful condi-
also serve as a protective mechanism against stress. In tions, exogenous CRH causes enhanced behavioral
addition, feed intake, appetite, and other catabolic and responses. Neuropeptides prepare the animal to perceive
anabolic processes are altered in response to stress. stimuli and cause an animal to behave a certain way,
Physiological responses to stressful situations are critical which enables it to respond appropriately to environ-
to the adaptability of the animal, butrepeatedexposureto mental changes. Other neuropeptides are probably in-
stressorsoramassivesinglestressfulexperiencemaylead volvedinthebehavioralresponsestostress,butfewhave
topathologicalconsequences. been described at this time.
CONCEPT OF ALLOSTASIS
BEHAVIORAL RESPONSES
A new concept called allostasis has evolved in order to
Stress elicits a broad range of behavioral responses in encompass the various degrees and outcomes of stress
which the profileis dependent uponcharacteristics ofthe responses across species. Allostasis is a process that sup-
organism (i.e., coping ability, dominance order) and the ports homeostasis in which stability is achieved through
stressor (i.e., severity, duration). Most often these change. Thus, the physiological parameters change as
behaviors are indicative of fear and anxiety. Animals environments and other life history stages change. Allo-
frequently exhibit decreases in exploratory activity and stasis involves the whole brain and body and is regulated
social interaction while exhibiting increases in locomotor by the brain’s attempt to alter and sustain behavioral and
activity, vocalization, and inappropriate behaviors (e.g., physiological adjustments in response to changing envi-
stereotypies) in response to stressors. Typically, stress ronments and challenges. Thus, the concept of allostasis
causes changes in normal behaviors instead of causing incorporates the adaptive function of regulating homeo-
new behaviors. In general, behavioral adjustments to kinetic responses to the pathological effects of the in-
stress are adaptive in nature. It has been suggested that ability to adapt.
at the onset or during mild bouts of stress, behavioral An allostatic state leads to an imbalance of the
adjustments can modulate the animal back to ‘‘normal’’ primary mediators of allostasis (i.e., glucocorticoids,
without eliciting a physiological response. During mild catecholamines), overproduction of some and underpro-
thermal stress one can only detect behavioral adjust- duction of others. Allostatic load is the cumulative
ments in response to thermal stress (end of the comfort effect of an allostatic state. Allostatic load can increase
zone), which may be enough to help the animal cope. In dramatically if additional loads of unpredictable events
fact, it’s not until the thermal environment changes in the environment occur in addition to adaptive
further that the animal requires measurable behavioral responses to seasonal or other demands. In essence,
and physiological adjustments. Despite these adjust- the mediators of allostasis are protective and adaptive,
ments, the homeokinetic responses are within normal thus increasing survival and health. However, they can
range. Essentially, it’s not until the animal experiences be damaging.
8 AdaptationandStress:Neuroendocrine,Physiological,andBehavioralResponses
1. Chrousos, G.P.; Gold, P.W. The concepts of stress
system disorders: Overview of behavioral and physical
homeostasis. J. Am. Med. Assoc. 1992, 267 (9), 1244
1252.
2. Moberg, G.P. Biological Response to Stress: Implications
for Animal Welfare. In The Biology of Animal Stress;
Moberg, G.P, Mench, J.A., Eds.; CABI Publishing: New
York, 2000;1 21.
3. McEwen,B.S.;Wingfield,J.C.Theconceptofallostasisin
biologyandbiomedicine.Horm.Behav.2003,43(1),2 15.
4. McGlone, J.J. What is animal welfare? J. Agric. Environ.
Ethics 1993, 6,26 36.
5. Schulkin, J. Allostasis: A neural behavioral perspective.
Horm.Behav. 2003,43(1),21 27.
6. Koob, G.F.; LeMoal, M. Drug addiction, dysregulation of
reward,andallostasis.Neuropsychopharmacology2001,24
(2),97 129.
CONCLUSION REFERENCES
Intermsofshort-termgoals,thestressresponseinitiatedby
aparticularstressorprovidesaseriesofhomeostaticmech-
anismsaswellasbehavioralandphysiologicaladaptations.
On the other hand, allostasis enables an organism to main-
tain physiological and behavioral stability despite adverse
and fluctuating environmental conditions. The responses to
stress involve numerous endocrine and neural systems that
contributetoorchestratingdefensesthatenabletheanimalto
adapt and maintain behavioral and physiological stability.
Behavioralandphysiologicalprocessesworkinconjunction
toregulatetheviabilityoftheinternalmilieu.Duringacute
stress, the biological cost to an animal is minimal, but
maximal during chronic stress. The inability to initiate an
appropriate and adequate stress response can be highly
deleterious,thusaffectinghealthandreproduction,whichin
turn impacts survivability and well-being.
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EncyclopediaofAnimalScience 9
DOI:10.1081/EEAS120019428
Copyright 2005byMarcelDekker,Inc.Allrightsreserved.
TexasA&M University,College Station,Texas, U.S.A.
Guoyao Wu
Jon Tate Self
INTRODUCTION tryptophan determination because of its relative stability.
Both acid and alkaline hydrolysis are accompanied by
Anamino acidcontainsbothamino andacidgroups.The deamination of glutamine and asparagine.
names for amino acids are largely derived from Greek
(e.g., glycine from the Greek word ‘‘glykos,’’ meaning
sweet).Over300aminoacidsoccurinnature,butonly20
serve as building blocks of proteins. Amino acids are AMINO ACID METABOLISM
substrates for the synthesis of many biologically active
substances (including NO, polyamines, glutathione, Amino Acid Synthesis
nucleic acids, hormones, creatine, and neurotransmitters)
that regulate metabolic pathways essential to the life and Microorganisms in the digestive tract can synthesize all
productivity of animals. Their abnormal metabolism amino acids in the presence of ammonia, sulfur, and
disturbs whole-body homeostasis, impairs animal growth carbohydrates. All animals can synthesize tyrosine as
and development, and may even cause death. Thus, well as the following amino acids and their carbon
knowledge of amino acid biochemistry and nutrition is skeletons: alanine, asparagine, aspartate, cysteine, gluta-
of enormous importance for both animal agriculture mate, glutamine, glycine, proline, and serine. The ability
and medicine. to synthesize citrulline and its carbon skeleton varies
amongspecies,butargininecanbemadefromcitrullinein
all animal cells.
Because of its large mass (representing 45% of adult
AMINO ACID CHEMISTRY bodyweight),skeletalmuscleaccountsforthemajorityof
glutamine and alanine synthesis from branched-chain
Except for glycine, all amino acids have an asymmetric amino acids (BCAA) in animals. These synthetic path-
carbon and exhibit optical activity. The absolute ways also occur in extrahepatic tissues, including the
configuration of amino acids ( - or -isomers) is defined brain,adiposetissue,intestine,kidney,lung,placenta,and
with reference to glyceraldehyde. Except for proline, lactating mammary gland. The liver and kidney are the
all protein amino acids have both a primary amino group major sites for the synthesis of tyrosine from phenylala-
and a carboxyl group linked to the a-carbon atom nine by phenylalanine hydroxylase, whereas hepatic
(hence a-amino acids). In b-amino acids (e.g., taurine transsulfuration is primarily responsible for cysteine syn-
and b-alanine), an amino group links to the b-carbon thesisfrommethionine.Thereisnoconversionoftyrosine
atom. Posttranslationally modified amino acids (e.g., into phenylalanine or cysteine into methionine. In con-
4-hydroxyproline, 5-hydroxylysine, 3-methylhistidine, trast, there is reversible interconversion of serine into
and dimethylarginines) occur in some proteins. The glycinebyhydroxymethyltransferaseintissues,including
biochemical properties of amino acids vary because of theliver,kidney,lactatingmammarytissue,placenta,and
their different side chains. The amino and acid groups of intestine. Proline can be synthesized from arginine in
allaminoacidsarecompletelyionized(zwitterionicform) animal cells containing mitochondria, and from gluta-
at physiological pH. mine and glutamate in most mammals (e.g., pigs and
Amino acids are stable in aqueous solution at ruminants).
physiological temperature, except for glutamine, which Utilization of precursors for the synthesis of -amino
is slowly cyclized to pyroglutamate (<2%/day at 1 mM), acids is of practical importance in animal production.
andcysteine,which undergoes rapidoxidation tocystine. Most -amino acids, except for -lysine, -threonine,
Acid hydrolysis of protein results in almost complete -cystine, -arginine and -histidine, can be converted
destruction of tryptophan, the oxidation of cysteine to into -amino acids in animals via widespread -amino
cystine, and some degradation of methionine, serine, acid oxidase and transamination. The efficiency of
threonine, and tyrosine. Alkaline hydrolysis is used for -aminoacidutilization,onamolarbasisofthe -isomer,
Amino Acids: Metabolism and Functions