Pulse radiolysis studies of WR-1065

Abstract

WR-1065, the dephosphorylated sulfhydryl form of WR-2721 has been suggested as the metabolite responsible for the radioprotective abilities of the latter and as being active during the radiation chemical stage of damage production. Hence this study was performed to determine some of the radiation chemical parameters of the compound. Pulse radiolysis techniques, developed for use with SH compounds, were employed in the current work: Three systems were used to attempt to measure the rate constant for reaction of OH' radicals with WR-1065; (a) Competition with phenylalanine, in which the decrease in the amount of RSSR- caused by the addition of phenylalanine is monitored; this failed because of the absence of an absorbance attributable to RSSR-. (b) Competition with 2,2′-Azinobis-(3-ethylbenzthiazo6ne-6-sulfonate) (ABTS~ monitoring the decrease in OH' induced ABTS absorbance caused by WR-1065 addition. This was unsuccessful due to reaction of RS with ABTS. (c) Competition with CNS −, observing the decrease in (CNS) 2 −caused by WR-1065 addition indicates that the second order rate constant for reaction of OH' with WR-1065 is 9.2 ± 03 X 109 M −1 s −1 To investigate the ability of WR-1065 to react with DNA radicals the yield of ABTS radicals was monitored in a situation where DNA scavenges 50% of the OH' radicals in the presence of ABTS (20°6) and WR-1065 (3096). DNA radicals were shown not to react with ABTS but WR-1065 radicals do, the absence of additional ABTS absorbing species indicates that DNA radicals do not react with WR-1065 under the experimental conditions used. An attempt was made to investigate the possibility that WR-1065 is concentrated close to DNA macromolecules in solution: The rate constant for reaction Br 2 staggered− with WR-1065 was measured in the presence and in the absence of excess DNA (with which Br 2 staggered− was not observed to react). The observed rates of reaction were independent of the presence of DNA indicating that the latter does not affect the availability of WR-1065 for reaction with Br 2 staggered− radical. Reaction of WR-1065 with Br 2 − was used as a probe for determining the pK of the SH group. The rate of reaction increases from 1.8 × 10 8 M −1 s −1 at pH 6.3 to 1.6 × 109 M −1 s −1 at pH 8.4, the mid-point of the increase indicates a pK of 7.3.