Serine Transhydroxymethylase SUBUNIT STRUCTURE AND THE INVOLVEMENT OF SULFHYDRYL GROUPS IN THE ACTIVITY OF THE ENZYME

Abstract The results of peptide map patterns, end group analysis, and disc electrophoresis experiments in gels containing 8 m urea show that serine transhydroxymethylase from rabbit liver consists of four identical polypeptide chains. Evidence is presented to show that serine transhydroxymethylase can exist in an oxidized form and a reduced form. The oxidized form is about 60% as active as the reduced form but can be converted to the reduced form by incubation with a sulfhydryl compound, such as dithiothreitol, and pyridoxal 5'-phosphate. In the absence of a sulfhydryl compound the reduced enzyme is converted to the oxidized enzyme over a period of several weeks at -5°. The apoenzyme is converted to the oxidized form in a few hours. The reduced enzyme has 12 sulfhydryl groups which react with 5,5'-dithiobis(2-nitrobenzoic acid). Kinetically they are of two classes. Eight of these sulfhydryl groups react rapidly and four more slowly. The oxidized enzyme has only eight sulfhydryl groups which react with 5,5'-dithiobis(2-nitrobenzoic acid), four reacting rapidly and four more slowly. The reaction of the enzyme with 5,5'-dithiobis(2nitrobenzoic acid) causes dissociation of pyridoxal 5'-phosphate and complete loss of enzymatic activity