Polyglutamyl folate coenzymes and inhibitors of chicken liver glycinamide ribotide transformylase

Abstract

Chicken liver glycinamide ribotide transformylase (5,10-methenyltetrahydrofolate:5'phosphoribosylglycinamide formyltransferase, EC 2.1.2.2), an enzyme of purine biosynthesis de novo, has greater specificity for its poly-γ-glutamyl folate coenzymes, 5,10-methenyltetrahydropteroyl(glutamate) n, where n = 3, 4, 5, 6 or 7, when compared to the monoglutamyi folate coenzyme. The relative specificity constants ( V/ K m) for the coenzymes 5,10-methenyltetrahydropteroyl(glutamate) n are 1.0, 1.6, 2.6, 2.4, 2.4, 4.9 and 1.5 for n=1, 2, 3, 4, 5, 6 and 7, respectively. Pteroylpoly-γ-(glutamate) n, where n=3, 4, 5, 6 or 7, are much better inhibitors of this enzyme when compared to the pteroylmonoglutamate. The concentration of inhibitor required for 50% inhibition was found to be 490, 120, 58, 28, 16, 14 and 12 μM for n=1, 2, 3, 4, 5, 6 and 7, respectively. Inhibitors with four or more glutamic acid residues gave grossly non-linear Dixon plots, in contrast to the linear plots obtained using inhibitors with three or less glutamic acid residues. The above findings make it feasible for the activity of glycinamide ribotide transformylase to be regulated by alteration in the length of the poly-γ-glutamyl chain of its folate coenzymes and of folate inhibitors.