Sulfhydryl studies of spinach leaf glyoxylic acid reductase.

Spinach leaf glyoxylic acid reductase, an enzyme containing two identical or similar subunits, has been shown to have 12 half-cystine residues. Four of these are necessary for catalytic function and can be titrated as free sulfhydryl groups. One mole of cysteinyl residues per mole of enzyme is essential to catalytic activity when evaluated under conditions favoring optimal activity. All free sulfhydryl groups are located within a single peptide when peptide maps of tryptic hydrolysates are examined. Cooperative function of the free sulfhydryl groups is indicated and substrate protection from sulfhydryl reagents is shown. Two active sites and a close spatial relationship of the free sulfhydryl residues are suggested by these data.