Report of the U.K. Government's Advisory Committee on Irradiated and Novel Foods

Summary

Oxidation processes of radiation-generated three-electron-bonded intermediates derived from methionine Met₂ and Met[S∴X] (X = Cl, Br) were investigated through reaction with tryptophan and tyrosine, using the optical pulse radiolysis method. Bimolecular rate constants have been measured for the reactions Met₂ with TrpH(k = 3·8 × 10⁸ dm³ mol^?1 s^?1 and k = 4·9 × 10⁸ dm³ mol^?1 s^?1 at pH 1 and 4·3, respectively) and Met₂ with tyrosine, k = 3·8 × 10⁷ dm³ mol^?1 s^?1. Rate constants for intermolecular transformation of Met[S∴Br] and Met[S∴Cl] into TrpH^+· or Trp· were also estimated. They varied from 4·7 × 10⁸ dm³ mol^?1 s^?1 (bromide species) to 1·0 × 10⁹ dm³ mol^?1 s^?1 (chloride species). It has also been established that azide radicals , N·₃ in contrast to dihalide radicals do not form transients of Met[S∴X] (X = N₃)-type. However, oxidation of N^?₃ by Met₂ occurs with a bimolecular rate constant of 2·8 × 10⁸ dm³ mol^?1 s^?1. These results are discussed in the light of some equilibria which have been proposed earlier for methionine-halide systems.     

Intramolecular electron transfer in peptides containing methionine, tryptophan and tyrosine: a pulse radiolysis study

The kinetics of pulse radiolytically induced intramolecular radical transformations: Trp/N.----Tyr/O., Tyr/O.----Trp/N., Met/S therefore Br----Trp/N. and Met/S therefore Br----Tyr/O. has been investigated at various pH levels and temperatures in model peptides: Trp-(Pro)n-Tyr, Trp-(Gly)n-Tyr, Trp-(Pro)n-Met (n = 0-3), Tyr-Phe-Met-Arg-Phe-NH2 x 2AcOH, Met5-enkephaline and [D-Ala]2Met5-enkephaline. The rate constants of the Trp/N.----Tyr/O. transformation at pH 8 were found to decrease exponentially with the distance between Trp and Tyr in the proline peptides, while in the glycine peptides the rate of the transformation is less dependent on the number of bridging glycine residues. The activation energies determined fall into the range 10-20 kJ mol-1 irrespective of: (i) the ionization state of tryptophyl radical and tyrosine, (ii) type of bridging amino acids, and (iii) reversal of the direction of the electron transfer upon tyrosine OH group ionization. The activation entropies at 298 K for the peptides of the glycine and proline series are negative and rather high, and fall into the relatively narrow range of -90 to -140 J mol-1 deg-1. These activation parameters seem to indicate that a tunnelling mechanism is involved in the electron transfer between strictly oriented aromatic moieties of Trp and Tyr. The variation of the activation parameters with average separation distance in the case of Trp-(Pro)n-Tyr shows a predominance of the electronic factor over the nuclear in determining the distance dependence of the electron transfer rate. The intramolecular Met/S therefore Br----Tyr/O. transfer proceeds with the rate constant of 1.1 x 10(5) s-1 in Met5-enkephaline and 5.7 x 10(4) s-1 in [D-Ala]2Met5-enkephaline. The activation parameters for this transformation Ea = 30 kJ mol-1 and delta S298 = -62 J mol-1 deg-1 are close to those of the Trp/N.----Tyr/O. transformation, suggesting a similar mechanism for the electron transfer.     

Interactions of Thiyl Free Radicals with Oxygen: A Pulse Radiolysis Study

Summary

A pulse radiolysis study of glutathione in aqueous solution at pH 5·5 containing N₂O/O₂ mixtures at various ratios indicates that oxygen rapidly adds to the thiyl glutathione radical yielding a transient absorption, with a maximum at 540 nm, whose characteristics appear to be compatible with assignment to the GSOO^· radical. The reaction appears to be an equilibrium whose kinetic constants have been estimated (k_f = 2·0 × 10⁹ dm³ mol^?1 s^?1, k_b = 6·2 × 10⁵ s^?1). Evidence for electron transfer from ascorbate to the GSOO^· radical has been obtained and the respective rate constant has been determined to be 1·75 ± 0·15 × 10⁸ dm³ mol^?1 s^?1.     

Chemical Properties of Water-soluble Porphyrins. 5. Reactions of Some Manganese (III) Porphyrins with the Superoxide and Other Reducing Radicals

Summary

Solution properties of three manganese porphyrins, in monomeric form, were investigated. These were the ‘picket-fence-like’ porphyrin Mn(III)-α,α,α,β-tetra-ortho(N-methylisonicotinamidophenyl)porphyrin (Mn(III)PFP) and two ‘planar unhindered’ porphyrins, the Mn(III)TMPyP (tetrakis (4-N-methylpyridyl)porphyrin) and Mn(III)TAP (tetra(4-NNN-trimethyl-anilinium)porphyrin). The porphyrin properties studied were: the absorption spectra in their manganic and manganous forms; acid/base properties of the aquo complexes; the effect of potential axial ligands (up to a concentration of 0·1 mol dm^?3) and their one electron reduction potentials. Knowing these properties, the reaction of the Mn(III) porphyrins with the superoxide radical and other reducing radicals were studied using the pulse radiolysis technique. The second-order reaction rate constant of O^?₂ with the Mn(III) porphyrins, which governs the catalytic efficiency of the metalloporphyrins upon the disproportionation of the superoxide radical, was 5·1 × 10⁷ to 4·0 × 10⁵ dm³ mol^?1s^?1, depending on the pH and the nature of the metalloporphyrin. These values are at least one order of magnitude lower than found for Fe(III)TMPyP. One electron reduction of the three Mn(III) porphyrins by e^?_aq, CO^?₂, CH₂OH and (CH₃)₂COH had similar second-order rate constants (10⁹-10¹⁰ dm³ mol^?1 s^?1). That for (CH₃)₂(CH₂)COH was about 10⁵ dm³ mol^?1 s^?1. Reduction in all cases produced the corresponding Mn(II) porphyrin and no intermediate was found. The oxidation reaction of the Mn(II) porphyrins by O^?₂ was approximately two orders of magnitude faster when compared to the reduction of Mn(III) porphyrins with the same radical. Since the reactivities of O^?₂ towards the three manganese (III) compounds follow their reduction potentials, it is suggested that these reactions are governed by an outer-sphere mechanism. This suggestion is corroborated by the finding that water molecules acting as axial ligands, in these aqueous solution systems, are not replaced by another potential ligand when the latter is in the concentration range of 100 mm or less.     

Subject Index

Summary

Using conductivity detection, pulse radiolysis experiments showed that solvent protonation of the electron adducts of cytosine, 5-methyl cytosine and 2′-deoxycytidine occurs with rate constants k ≥ 2 × 10⁴M^?1s^?1. The protonated electron adducts transfer an electron to p-nitroactetophenone (PNAP) with rate constants ranging from 3·5×10⁹ to 5·3·10⁹M^?1s^?1. The transfer is quantitative (G = 2·7), as shown by conductometric and spectroscopic measurements. In the presence of O₂ no electron transfer to O₂ takes place, implying that O₂ adds to the protonated electron adduct radicals.

No electron transfer from the H- and OH-adducts of the cytosine derivatives, either to PNAP or to O₂, takes place near neutral pH. It is suggested that the differences in the reaction behaviour of the H-adduct radicals and the protonated electron adduct radicals towards PNAP can be accounted for if different radicals are formed by H-addition and protonation of the electron adduct. The H atoms most probably add to the C-5-C-6 double bonds, whereas the electron adducts are protonated at N-3 and/or 0-2.     

Causation of Cancer. British Medical Bulletin,Vol. 14, No. 2

Summary

Peroxyl radicals of poly(U), poly(A), and single- and double-stranded DNA have been produced by photolysing H₂O₂ in oxygenated aqueous solution in presence of the substrates. The peroxyl radicals are formed by the reaction of OH radicals with the polynucleotides followed by addition of oxygen.

The lifetime of the peroxyl radicals and the rate constant of their reactions with the thiols cysteamine, gluthathione and dithiothreithol have been measured by time-resolved e.s.r. spectroscopy. The unusually long lifetimes range from 0·2 to 3·3 s. The activation energy for the decay for all four substrates is 10·3 ± 1 kcal/mol (43 kJ mol^?1). The reaction rate constants with the thiols range from k = 0·8 × 10⁴ to 1·3 × 10⁵ dm³ mol^?1 s^?1. The reactions of the thiols with the peroxyl radical of poly(U) are known to prevent strand break formation. This shows that the peroxyl radicals of poly(U) observed by e.s.r. are intermediates in the pathway leading to strand break formation.     

Chemical Properties of Water-soluble Porphyrins. 4. The Reaction of A ‘Picket-fence-like’ Iron (III) Complex with the Superoxide Oxygen Couple

Summary

Solution properties of the iron-(III) ‘picket-fence-like’ porphyrin, Fe(III)-α,α,α,β-tetra-ortho (N-methyl-isonicotinamidophenyl) porphyrin, (Fe(III)PFP) were investigated. These were acid/base properties of the aquo complex with pK_a of 3·9 and its aggregation (formation of dimer with K = 1 × 10^?10 dm³ mol^?1), complex formation with cyanide ions and 1-methyl imidazole (1-MeIm), spectral properties of the three iron complexes in their ferric and ferrous form and the one-electron reduction potential of these complexes. Knowing these properties, the reaction of the ferric complexes, aquo, dicyano and bis (1-MeIm), with the superoxide radical and other reducing radicals were studied using the pulse radiolysis technique. The second-order reaction rate constant of O^?₂ with the iron (III) aquo complex which governs the catalytic efficiency of the metalloporphyrin upon the disproportionation of the superoxide radical was 7·6 × 10⁷ dm³ mol^?1 s^?1, two orders of magnitude faster when compared to the reaction of each of the other complexes. The reduction by other radicals with all iron (III) complexes had similar second-order rate constants (10⁹ to 10¹⁰ dm³ mol^?1 s^?1). The reduction reaction in all cases produced Fe(II)PFP and no intermediate was found. The oxidation reaction of Fe(II)PFP by O^?₂ was one order of magnitude faster when compared to the reduction of Fe(III)PFP by the same radical. Since the reactivity of O^?₂ toward the three iron (III) porphyrin complexes follows their reduction potentials, it is suggesting the formation of a peroxo Fe(II) porphyrin as an intermediate. The reactions of the Fe(II)PFP complexes with dioxygen were also studied. The aquo complex was found to be first order in O₂ and second order in Fe(II)PFP, suggesting the formation of a peroxo Fe(II) porphyrin as an intermediate. The intermediate formation was corroborated by evidence of the rapid CO binding reaction to the aquo complex of Fe(II)PFP. The two other complexes reacted very slowly with O₂ as well as with CO.     

The Reactions of the Hydroxymethyl Radical with 1,3-dimethyluracil and 1,3-dimethylthymine

Summary

Hydroxymethyl radicals ·CH₂OH, generated by the radiolysis of methanol (0·5 mol dm^?3) in N₂O-saturated aqueous solutions, were reacted with 1,3-dimethyluracil or 1,3-dimethylthymine (10^?3) mol dm^?3). The products were identified and their G values determined. It has been concluded that in 1,3-dimethyluracil ·CH₂OH attack occurs only at C(6) while in 1,3-dimethylthymine there is partitioning between addition (two-thirds) and H-abstraction from the C(5)-methyl group (one-third). A rate constant for CH₂OH addition to 1,3-dimethyluracil of about 10⁴ dm³ mol^?1 s^?1 is estimated. Complexities that may arise in the radiolysis of pyrimidines such as 1,3-dimethylthymine, apparently as a consequence of the formation of 5-alkylidenepyrimidines, are discussed. A value of 0·15 has been estimated for the disproportionation/combination ratio for the hydroxymethyl radical self-termination reaction.     

Mechanism of Reduction of Bleomycin-Cu(II) by CO−2 and Oxidation of Bleomycin—Cu(I) by H2O2 in the Absence and Presence of DNA

Summary

The reduction reaction of bleomycin—Cu(II) by CO^?₂ has been studied by γ and pulse radiolysis at pH7. The CO^?₂ radical reduces bleomycin—Cu(II) at a rate of (6·7 ± 0·7) × 10⁸ dm³ mol^?1 s^?1. In the presence of calf thymus DNA the rate of the reduction decreased as the concentration of DNA increased, indicating that the reduction reaction proceeds through free bleomycin—Cu(II). The stoichiometry and the kinetics of the oxidation of bleomycin—Cu(I) by H₂O₂ in the presence and absence of DNA have been studied. Our observations suggest that the OH^· radical is not produced during this reaction and the degradation of the drug occurs in the absence and presence of DNA. We assume that bleomycin—Cu(II) in the presence of a reducing agent and molecular oxygen or H₂O₂ does not cleave DNA since the oxidizing species, which are formed during the oxidation reaction by H₂O₂, attack the drug even in the presence of DNA.     

Pulse Radiolytic Study of the Interaction of SO−·4 with Deoxynucleosides. Possible Implications for Direct Energy Deposition

Summary

Using the technique of pulse radiolysis it has been demonstrated that the interaction of SO^?·₄ with deoxynucleosides (k ～ < 2 × 10⁸–2·3 × 10⁹ dm³ mol^?1 s^?1) in aqueous solution at pH 7·0 results in the formation of the corresponding one-electron oxidized radicals which either deprotonate or hydrate to yield OH adducts. Based upon the ease of oxidation of the deoxynucleosides, dG, dA, dC, dT, by SO^?·₄, the apparent redox potentials are in the order dG ? dA ≈ dC > dT. With the exception of deoxyuridine, the deoxynucleoside radicals produced on interaction with SO^?·₄ have been shown to have oxidizing properties based upon the interactions with tetranitromethane and the nitroxyls, TMPN and NPPN. The deoxynucleoside radicals (dG, dA and dC) do not interact with oxygen (k < 10⁶ dm³ mol^?1 s^?1) in contrast to the interaction observed with the thymidine radical (k = 2·5 × 10⁷ dm³ mol^?1 s^?1). The implications of these findings are presented in terms of the properties of the discussed radicals as relating to those of potential DNA base radicals (positive centres) produced by direct energy deposition within DNA. The use of SO^?·₄ to mimic, to some extent, the effects of direct energy deposition in DNA may assist in our understanding of the resulting molecular processes relevant to radiobiological studies.     

Free Radical Inactivation of Lactate Dehydrogenase

The enzyme lactate dehydrogenase (LDH) has been irradiated under various conditions to assess the relative contributions of ·H, ·OH, H₂O₂ and ·O₂^? to LDH inactivation, and it is concluded that ·OH is the only important inactivating species. Further the effect of the selective free radicals, ·(SCN)₂^?, ·Br₂^? and ·I₂^? on the activity has been studied. In neutral solution, the order of inactivating effectiveness is ·I₂^? > ·OH > ·Br₂^? > ·(SCN)₂^?. At pH 8·6, ·OH and ·Br₂^? are approximately equal in effectiveness, whereas ·(SCN)₂^? is the least efficient. The radiation inactivation of LDH is accompanied by a loss of sulphydryl groups, and it is suggested that the primary target for radiation damage in LDH is the active site cysteine-165. Subsequent conformational changes are suggested to account for the apparent loss of coenzyme-binding ability and changes in the enzyme's kinetic parameters. The effect of bound coenzyme (NAD) on radiation-induced inactivation of N₂O and air-saturated solutions was also investigated, and it is shown that NAD binding protects LDH.     

Reactions of OH Radicals with Poly(U) in Deoxygenated Solutions: Sites of OH Radical Attack and the Kinetics of Base Release

Summary

Pulse radiolysis of N₂O-saturated solutions of poly(U) in the presence of tetranitromethane showed that 81 per cent of the radicals formed are reducing in nature. Using data from other sources it has been estimated that 70 per cent of the OH radicals add to the base at C(5) and 23 per cent at C(6) while only 7 per cent abstract an H-atom from the sugar moiety. To a large extent the C(5) OH adduct radicals attack the sugar moiety of poly(U) thereby inducing strand breakage and base release. G (base release) = 2·9 can be subdivided into three components: (a) immediate (20 per cent), (b) fast (50 per cent) and (c) slow (30 per cent). The immediate base release must occur either during the free-radical stage or as a result of the rapid (t_½ < 4 min at 0°C) decomposition of a diamagnetic product. The fast and the slow processes are only readily observable at elevated temperatures, e.g. at 50°C the half lives are 83 min and 26 h, respectively (E_a (fast) = 68 kJ mol^?1, E_a (slow) = 89 kJ mol^?1, A (fast) = 1·5 × 10⁷ s^?1, A (slow) = 1·9 × 10⁹ s^?1. It is concluded that there are three different types of sugar lesions giving rise to base release, structures for which are tentatively proposed.     

A Longitudinal Study of Cardiovascular Stability in Active Men Aged 45 to 65 Years

In brief: Cross-sectional studies of physical performance generally show a linear decline in maximum aerobic power (V O₂ max) with increasing age (about 1% to 2% per year). In the study described in this article, 15 men were serially followed for 20 years. They performed exercises consisting of walking, running, swimming, and cycling for an average of 3.6 days per week and requiring an energy expenditure of 2,104 kcal per week. Over the 20 years, directly measured V O₂ max declined 12% (from 44.4 to 38.9 ml· kg^?1· min^?1, or 0.27 ml· kg^?1· min^?1 per year). Essentially no differences were seen in resting heart rate or arterial blood pressure. The authors conclude that physical training forestalls the decline in V O₂ max until at least age 65.     

Inactivation of Deoxyribonuclease I in Aqueous Solution by Thermal Neutrons

Summary

Degradation of DNA when γ-irradiated in aqueous solutions containing cysteine can be efficiently enhanced not only with oxygen, but to the same extent also with Cu²⁺ ions under hypoxic conditions. The result can be explained by ‘self-repair’ in this system due to recombination of DNA^· with RSS^·?R intermediates, and repair inhibition by oxygen or copper involving RSS^·?R scavenging. It is emphasized that oxygen enhancement in DNA–thiol systems may occur not only by peroxidation, via defect fixation (DNA–O^·₂) or thiol activation (RS–O^·₂), but also by the well-established inactivation of RSS^·?R by oxygen. There is evidence also from literature data for a correlation between oxygen enhancement and RSS^·?R stability, which varies with thiol concentration, pH and thiol structure.     

Aerobic Requirements for Moving Handweights Through Various Ranges of Motion While Walking

In brief: This study compared the aerobic metabolic requirements of normal walking (without handweights and with normal arm motions) with requirements of walking while pumping 1-,2-, or 3-lb handweights through Various ranges of motion. Nine male subjects Walked with and without handweights at speeds of 1.12 to 1.79 m· sec^?1. Adding hand-Weighted arm movements significantly increased the oxygen consumption (V O₂) of normal walking by 2.1 to 25.5 ml· kg^?1· min^?1. The V O₂ for handweighted walking ranged from 17 to 43 ml· kg^?1· min^?1, or 113% to 255% of normal walking requirements at any given speed. These results indicate that walking while moving handweights through large ranges of motion provides a combined upper and lower body aerobic stimulus that is sufficient for endurance training for persons with poor to excellent levels of aerobic fitness.     

Kinetics of Radiation-induced Strand Break Formation in Single-stranded Pyrimidine Polynucleotides in the Presence and Absence of Oxygen; a Time-resolved Light-scattering Study

Summary

Time-resolved reductions in the light-scattering intensity (LSI) of aqueous oxic and anoxic solutions of poly-C and poly-U at pH 7·8, following pulse-irradiation, have been studied as indicies of strand break formation. With doses of 3–24 Gy per pulse, a number of kinetically distinct strand breakage components have been detected. A comparison of the LSI responses obtained from irradiations conducted under N₂O with those conducted under air or O₂ show no marked difference in the overall extent of LSI change. However, the immediate and fast (t_½ ≤ 50 μs) reduction in LSI, accounting for about 18–19% of the pyrimidine polynucleotide's total LSI response in oxic solution, is reduced in the absence of oxygen, to about 12% of the total LSI response found with poly-C and to about 9% for poly-U. For poly-C there is a five-fold enhancement in the rate of major strand breakage under anoxia [k₁(N₂O) = 7·9 s^?1] whereas for poly-U a more modest enhancement (about two-fold) is observed. These enhanced rates are mirrored by those for the losses of the principal optical anoxic absorptions (observed pulse radiolytically) that are assigned to the pyrimidine 6-yl base radicals. Such findings support a proposal that the rate-limiting step of major strand breakage for pyrimidine polynucleotides is a base radical mediated hydrogen atom abstraction raction (Lemaire et al. 1987, Hildenbrand and Schulte-Frohlinde 1989). Irradiation of poly-C and poly-U in N₂O/O₂ (4:1, v/v) saturated solutions yields LSI changes much larger than those noted under N₂O and air (or O₂), which are in turn approximately double the responses observed under N₂. This indicates that the major strand breaking species of water radiolysis is the OH-radical and that there is an oxygen enhancement of single strand breakage of about 1·9 for poly-C and 1·6 for poly-U.     

Enhancement of Hyperthermic Cell Killing by Non-thermal Effect of Ultrasound

Summary

The present study was performed to elucidate the mechanism of enhanced hyperthermic cell killing by a non-thermal effect (cavitation and direct effect) of ultrasound under various gas conditions. Cavitation, as indicated by formation of DNA double-strand breaks and liberation of potassium iodide, was completely inhibited under N₂O-saturated conditions, while it was promoted under O₂-, Ar-, and N₂-saturated conditions. Mouse L cells were treated with ultrasound (1 MHz continuous wave, spatial peak temporal average intensity; 3·7 W/cm²) and/or 44°C hyperthermia in medium saturated with O₂, Ar, N₂ (with cavitation) or N₂O (with direct effect). The synergism on cell killing by ultrasound and 44°C hyperthermia was observed under N₂O-saturated conditions (enhancement ratio = 1·39). On the other hand, additive enhancement was observed under O₂-, Ar-, or N₂-saturated conditions. In addition, when cells were treated with 44°C hyperthermia before or after sonication under N₂O-saturated conditions, synergistic cell killing was not observed. These results suggested that the direct effect of ultrasound alone did not influence cell killing, but enhanced the hyperthermic cell killing synergistically, when both agents simultaneously acted on the cells.     

Model Studies for the Direct Effect of High-energy Irradiation on DNA. Mechanism of Strand Break Formation Induced by Laser Photoionization of Poly U in Aqueous Solution

Summary

Laser flash photolysis of polyuridylic acid (poly U) in anoxic aqueous solutions leads to biphotonic photoionization of the uracil moiety followed by the formation of single strand breaks (ssb). The rate constant for ssb formation (1·0 s^?1, obtained from the slow component of conductivity increase at 23°C and pH 6·8) increases with decreasing pH to 235 s^?1 at pH 3·5. The activation energy (pre-exponential factor) was measured to be 66 kJ mol^?1 (5 × 10¹¹ s^?1) at pH 6·8. Addition of dithiothreitol (DTT) or glutathione (GSH) prevents ssb formation by reacting with a poly U intermediate (rate constant = 1·2 × 10⁶ and 0·16 × 10⁶ dm³ mol^?1 s^?1, respectively). Since with OH radicals as initiators very similar data have been obtained for the kinetics of ssb formation and for the reaction with DTT, we conclude that photoionization of the uracil moiety in poly U leads eventually to the same chemical pathway for ssb formation as that induced by OH radicals. Furthermore, we propose that protection by DTT and GSH occurs via H donation to the C-4′ radicals of the sugar moiety of DNA and to the C-4′ and the C-2′ radicals of poly U.     

Photochemical Generation of Superoxide Radical and the Cytotoxicity of Phthalocyanines

Summary

The effect of the central metal atom on the photodynamic activity of phthalocyanine dyes has been estimated by cytotoxicity to cultured Chinese hamster cells. Chloroaluminium phthalocyanine, followed by the Zn- derivate, were found to be the only active dyes. In parallel it was found that visible light (615 ± 10 nm) excitation of phthalocyanines dissolved in dimethylsulphoxide in the presence of oxygen generates superoxide radical anion. O^?₂ radicals were spintrapped with 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) and identified by electron spin resonance. The quantum yields for O^?₂ generation range from 10^?5 (Zn-phthalocyanine) to 4·2 × 10^?4 (Ga-phthalocyanine). The efficiency of generating O^?₂ was apparently uncorrelated with the phototoxicity of the same dyes. Furthermore, the biological photodamage could not be inhibited by the addition of superoxide dismutase. It is concluded that O^?₂ is involved very little, if at all, in the phthalocyanine-induced photo-killing of mammalian cells.     

The Radiolysis of Dilute Solutions of Lysozyme

Summary

Pulse radiolysis was used for the study of transients formed from lysozyme solutions. Reactions of these transients with cysteine and oxygen were analysed kinetically. The rate constant for the reaction of lysozyme with e^?_aq was 5·2 × 10¹⁰ M^?1 sec^?1 and for lysozyme with OH was 3·1 × 10¹⁰ M^?1 sec^?1. For the reaction of the lysozyme transient (λ_max 400 mμ) with cysteine and oxygen, the rate constants were 4·1 × 10⁶ M^?1 sec^?1 and 3·38 × 10⁸ M^?1 sec^?1 respectively. The corresponding figures for the lysozyme transient (λ_max 310 mμ) were 4·6 × 10⁸ M^?1 sec^?1 and 1·6 × 10⁸ M^?1 sec^?1. These data are correlated with those obtained from steady-state radiolysis and elucidate an aspect of the known radiobiological oxygen effect.