Enzymes of the clostridial purine fermentation. Serine hydroxymethyltransferase

Serine hydroxymethyltransferase has been partially purified from lyophilized cells of Clostridium cylindrosporum. An apoenzyme free of pyridoxal phosphate was prepared by chromatography of the enzyme on DEAE-cellulose in the absence of the cofactor. The activity of the enzyme was dependent on the addition of catalytic amounts of pyridoxal phosphate in addition to the substrates. No compound was found that would substitute for l-serine in the enzymic reaction. (±)-Tetrahydropteroate and (-)-tetrahydropteroyltriglutamate, a naturally occurring form of the cofactor, can replace (±)-tetrahydrofolate in the reaction. The enzyme showed the maximum activity in media at pH values between 6.8 and 7.4. The enzymic product formed, 5,10-methylenetetrahydrofolate, was shown to have a positive sign of rotation. The enzyme is inhibited by pyridoxamine phosphate and the inhibition could be reversed by incubation with pyridoxal phosphate.