Effects of selenocystine and selenomethionine on activation of sulfhydryl enzymes

Abstract

Selenocystine increased the rate of activation, peak activity, and time-integrated activity of the sulfhydryl enzymes papain and glyceraldehyde-3-phosphate dehy-drogenase, with sulhydryl compounds. Selenocystine also increases the rate and extent of inactivation of ribonuclease with cysteine. The effects on activation of papain, of Selenomethionine, of varying selenoamino acid concentration, and of the presence of N 2, O 2, and EDTA are reported. A general mechanism of activation, involving catalytic participation of Selenocystine in sulfhydryl-disulfide exchange reactions is postulated. Papain activated in the presence of Selenocystine and Selenomethionine is partially protected against oxidative inactivation. The characteristics of the assay plots of substrate hydrolysis versus time, the effects of variation of assay parameters, and the dialysis of solutions containing protected papain indicate that the selenoamino acids bind reversibly to enzyme sulfhydryl groups to provide protection. The implications of the results are discussed in terms of the possible functions of selenium in biochemical systems.