A spectrophotometric study of the reaction catalysed by serine transhydroxymethylase

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Abstract
Serine transhydroxymethylase has been further purified five- to nine-fold by the use of ion-exchange columns. By this additional step the overall purification of the enzyme from rabbit-liver extracts is about 250-fold. The behaviour of the enzyme on ion-exchange resins indicates that it is a basic protein with isoelectric point above pH 7.7. The spectrophotometric evidence indicates that one isomer of DL-methylenetetrahydropteroylglutamic acid, produced by the action of formaldehyde on DL-tetrahydropteroylglutamic acid, is a substrate for serine transhydroxymethylase. Similar evidence has been obtained which indicates that only one isomer of tetrahydropteroylglutamic acid participates in the serine-transhydroxymethylase reaction. This isomer is formed by dihydropteroylglutamic reductase from dihydropteroylglutamic acid and in the presence of serine transhydroxymethylase into a product with an absorption spectrum identical with that of methylenetetrahydropteroylglutamic acid. Like methylenetetrahydropteroylglutamic acid the product is instantly converted into tetrahydropteroylglutamic acid in the presence of bisulphite. The equilibrium constants have been obtained for the serine-transhydroxymethylase reactions from spectrophotometric data.