Radioprotection: Chemical Compounds-Biological Means

84 Pages·1977·3.221 MB·English

by  J. D. Chapman, A. P. Reuvers (auth.), A. Locker, K. Flemming (eds.)

EXPERlENTIA SUPPLEMENTUM 27
Radioprotection
Chemical Compounds
Biological Means
A Symposium by Correspondence
Edited by A. Locker and K. Flemming
1977 Springer Basel AG
CIP-Kurztitelaufnahme der Deutschen Bibliothek
Radioprotection: chern. compounds,
biolog. means; a symposium by correspondence/
ed. by A. Locker and K. Flemming. - 1. Aufl
(Experientia: Supp!.; 27)
ISBN 978-3-0348-5584-6 ISBN 978-3-0348-5582-2 (eBook)
DOI 10.1007/978-3-0348-5582-2
NE: Locker, Alfred [Hrsg.)
All rights reserved
No part of this book may be reproduced in any form, by photostat, microfilm,
or any other means, without written permission from the publishers.
© Springer Basel AG 1977
Originally published by Birkhiluser Verlag Basel in 1977
Softcover reprint of the hardcover 1st edition 1977
Introduction
The highly topical problems of radioprotection will be treated in the following
issues of "Experientia" by competent experts in this field. Before printing the
papers have been exchanged among the authors to allow them for commenting
upon the articles so that a Symposium by Correspondence on Radioprotective
Means and Compounds arose. As organizers and editors of this symposium
acted A. LOCKER (Vienna) and K. FLEMMING (Freiburg, Br.).
Contents
9 J.D. Chapman and A.P. Reuvers: The Time-Scale of Radioprotection
in Mammalian Cells
19 J. Calkins: General Patterns of DNA Repair and their possible Signif
icance as Necessary Protection from Environmental Radiation Exposure
31 S. Homsey: Protection by Hypoxia and the Effect of Low Oxygen
Tensions on Radiosensitivity
45 L. Revesz and B. Littbrand: Radioprotection by Radiosensitizers
53 T. Sugahara, M. Horikawa, M.H. Ikita and N. Nagata: Studies on a
Sulfhydryl Radioprotector of Low Toxicity
63 J. M. Yuhas: Systemic Factors Affecting the Radioprotective Effec
tiveness of Phosphorothioates
71 C. Streffer: Studies on the Mechanism of 5-Hydroxytryptamine in
Radioprotection of Mammals
79 K. Flemming: Some Ideas Concerning the Mode of Action of Radio
protective Agents
87 A. Locker and K. Flemming: Some General Aspects of Radioprotection
(A Summary)
9
The Time-Scale of Radioprotection in Mammalian Cells
J.D. Chapman* and A.P. Reuvers
Medical Biophysics Branch, Atomic Energy of Canada Limited, Whiteshell Nuclear Research
Establishment, Pinawa, Manitoba ROE 110, Canada
Abstract
The chain of reactions, resulting in mutation and cell death, initiated
when ionizing radiation interacts with mammalian cells is complex and tra
verses a time-scale from a fraction of a picosecond to a few hours. Recently,
progress has been made in identifying some of the more important pathways
and the potentially damaging free radical intermediates. Protection can, in
principle, result from modification at several steps in these reaction chains,
each with its distinctive time-scale. To date we have demonstrated that radio
protection can be effected in mammalian cells by the modification of pro
cesses at three quite different times. In this paper, the temporal sequence of
radiation-initiated events are reviewed and the potential for or measurement
of radioprotection of each is discussed. Such an understanding of cellular
radioprotection has become possible by our improved understanding of
cellular radiation mechanisms derived from recent studies of its chemistry
[1-4].
Ionizations and Excitations in Cellular Molecules and Water
Cellular molecules and water are ionized and excited by interactions
with the photo- and Compton-electrons set in motion by the incident radia
tion. The loss of energy along the tracks of such energetic electrons is predom
inantly a function of the electron density of the bulk material and for living
cells and tissue the electron density does not vary significantly from that of
water. Ionizations, super-excitations, and excitations occur within 10-17 to
10-15 sec [5] and it is unlikely that these extremely fast processes could be
manipulated to result in a reduced initial yield of reactive species in mamma
lian cells. With cells one is always limited to those physical and chemical treat
ments, designed to modify cellular radiation response, which are non-toxic. To
date radioprotection of mammalian cells by modification of such initial events
has not been achieved and, in all likelihood, this timescale is not amenable for
cellular radioprotection.
* Present address (1976): Lawrence Berkeley Laboratory, University of California,
Berkeley, Calif., 97720, USA.
10 J.D. Chapman and A.P. Reuvers
Cell Inactivation by Direct Effect
The cellular molecules which constitute the radiation target have not
been identified, although there is mounting evidence which indicates that
DNA forms part of the target associated with the loss of cellular proliferative
capacity [6]. Since 20-300/0 of the cellular matter is other than water it might be
expected that at least 20-300/0 of radiation inactivation results from energy
deposited in this material. The discussion in the following section will indicate
that it is the energy deposited within 30 A of the cellular targets which results in
cell inactivation and not that deposited in the cellular bulk. Consequently, the
direct effect could constitute a greater or smaller proportion of the total
cellular inactivation depending upon the chemical specificity of the lesions
produced and their rep arability. Attempts to experimentally measure the
direct effect component of mammalian cell inactivation are subject to various
limitations, nevertheless values of between 30 and 400/0 have been reported
[7, 8]. The study of direct effect in cellular macromolecules in vitro indicates
that excitations and ionizations produced in these targets quickly lead to free
radical intermediates [4, 6]. By extending such in vitro studies to the cellular
situation we can write the direct reactions of radiation with cellular targets as,
T ~---T---,* --~) yu , (1)
T i~onization T+ +e-------~-'>. T' + H + +e- (2)
where T represents target molecule and the superscripts, * and a, represent
excited state and stable altered state, respectively. Again there is no
experimental evidence which suggests that the amount of direct effect in
mammalian cells occurring at these rapid time-scales can be reduced.
Indirect Effect of Cellular Water Molecules
Ifwe assume that cell inactivation results from radiation-induced altera
tions in some cellular target molecule(s), the energy deposited in the cellular
water could only lead to inactivation if transferred to these targets. Since cells
and tissue consist of 70-80% water, the energy deposited in this compartment
could be of importance.
Of the primary events initiated by radiation in water, the yields of ion
izations, super-excitations, and excitations are 3.45, 0.92 and 0.54, respectively
(in G-values) [9]. Excited and ionized water molecules undergo rapid molecu
lar reorganizations resulting in several reactive free radical species according
to the following reactions [10].
The Time-Scale of Radioprotection in Mammalian Cells 11
H20 ~ H20* ) H' +OH', (3)
H 0 ~ H 0++e-, (4)
2 2
H20++H20 ~ H30++OH', (5)
e-+H20 ~ H20- ~ H'OH-, (6)
e-+H+----) H' , (7)
e-+H20 ~ e;q. (8)
These reactions take place within very small volumes (average radius -15 A)
over the time-scale of 10-14 to 10-10 sec. Several different radical species
occupy each of these reactive volumes (or spurs) and recombination reactions
can occur producing H2 and H202. The overall reaction of radiation with
water can be written as
(9)
The products H' , OH', e;q, H2, H202 and H30 + are produced in water of
pH 7 at 10-9 to 10-8 sec with yields (G-values) of 0.6, 2.6, 2.6, 0.45, 0.75 and
2.6, respectively [10]. By this time, these species have diffused far enough away
from each other so that reaction with the cellular molecules becomes a more
likely event than recombination amongst themselves. Since some of these pro
ducts of water radiolysis produce cell inactivation (see below), it may be
possible to effect radioprotection by altering their initial yields. This would
involve competing in very fast processes (10-14 to 10-8 sec) and to date no
experimental evidence exists that indicates this is possible within mammalian
cells.
The free radicals OH' and e;q are produced in high yield and are
known to be highly reactive with several molecules of biological interest [11].
Recent studies on the chemical radioprotection of mammalian cells in vitro [8,
12-14] indicate that OH' are the most damaging of the water radicals. The
ability of various alcohols and dimethyl sulfoxide to protect aerated mam
malian cells against radiation inactivation was proportional to their reactivi
ties with OH' [14]. This one radical species was responsible for, at least, 60%
of the inactivation rate of aerated cells [8, 12]. Competition kinetic analysis of
these data indicate that the average lifetime of the OH' which produce cell
inactivation is between 10-9 and 10-8 sec. In Chinese hamster cells (V79), the
rate of reaction of cellular target(s) with OH' was found to be 9 x 108 sec-I
[14], indicating why such high concentrations of radical scavengers are
required to produce cellular radioprotection (Fig. 1). These lethal radicals
diffuse, on the average, only 15-30 A from their site of generation in cells to
react with the targets [8, 14]. This then is the fastest reaction initiated within
mammalian cells by radiation for which chemical radioprotection can be
12 J. D. Chapman and A. P. Reuvers
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~~'-~~ 4 Q.
Ii .00!l ......x.... ............... o ......... +~ :::-......... :::>Ii
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z~ .002 -e_~ e-e_ _e - O.l!! 0on
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It: 0 10.1 100 2 4 8 16 32 64 128
CONCENTRATION OF PROTECTOR [M]
Figure la Figure lb
Chinese hamster cell inactivation rate as a The reciprocal of the concentration effecting
function of concentration of various hydro· half-maximum protection plotted against the
xyl radical scavengers. The vertical bar absolute rate of reaction of each compound
through each curve indicates the concentra· with hydroxyl radicals. The symbols
tion effecting half-maximum protection. The represent the compounds as indicated in the
compound associated with the various sym legend of the left panel and the line has been
bols is indicated in the figure legend. drawn with a slope of 1.0.
demonstrated. The products of the reactions of OH' with biological molecules
are secondary radicals resulting predominantly from the addition of OH'
across a carbon-carbon double bond or from the abstraction of a hydrogen
from a carbon atom [2],
OH' + T-+ T-OH' , (10)
OH' +T-+H20+T', (11)
Hydrated electrons would also quickly react with the cellular target(s), Their
reactivities with various biomolecules, which could form part of the cellular
target(s), are, in general, higher than those of OH' [11], At 10 -9 to 10-8 sec, a
high yield of target radical anions would be expected in addition to those radi
cals generated by OH' attack.
(12)
Oxygen, which has an extremely fast reaction rate with e~q, is present in
aerated cells at a concentration of 270 JlM and could not effectively compete
with the target for e~q generated within a distance of25 A.