Lorentz Eldjarn

THE INACTIVATION OF SH ENZYMES BY X-RAYS

SummaryThe susceptibility of SH-enzymes to the indirect effect of ionizing radiation has been re-investigated. The claims of previous authors that these enzymes are particularly radiosensitive could not be confirmed. Dilute solutions of crystalline glyceraldehyde-3-phosphate dehydrogenase and yeast alcohol dehydrogenase were found to be inactivated by x-rays, with ionic yields of about 0·02. In contrast to previous reports no reactivation was obtained by the addition of reduced glutathione after the irradiation.The discrepancy between the present results and those of earlier investigators can in part be explained by the fact that the earlier estimations of ionic yields were based on molecular weights for the enzymes which differ by a factor of about 2 from those currently accepted. The present data seem to be consistent with a number of radiochemical and enzymological findings.

The mechanism of inactivation of penicillin by cysteine and other mercaptoamines.

Abstract The kinetics of penicillin inactivation by various mercaptoamines have been followed by polarimetry. In the case of primary mereaptoamines the reactions are of second order with respect to time and concentration of the reactants. By determining the appropriate rate constants under different conditions of pH and ionic strength the conclusion is reached that the rate limiting step is a bimolecular reaction involving the mercaptide ion. The mechanism probably involves the formation of a semimercaptol. Several possible mechanisms for the role of the amino group in the opening of the β-lactam ring are discussed. The inactivation of penicillin by the tertiary mercaptoamine N:N-dimethylcysteamine nearly follows first order kinetics and gives as end product a partially racemized penicilloic acid.