Folylpoly-.gamma.-glutamates as cosubstrates of 10-formyltetrahydrofolate:5'-phosphoribosyl-5-amino-4-imidazolecarboxamide formyltransferase

Abstract

N10-Formyltetrahydropteroylpoly-.gamma.-glutamates (N10-formyl-H4PteGlun) having N = 1, 3, 4, 5, 6 and 7 glutamyl residues were tested as cosubstrates of the purine biosynthesis enzyme 10-formyltetrahydrofolate:5''-phosphoribosyl-5-amino-4-imidazolecarboxamide formyltransferase [EC 2.1.2.3] (AICAR transformylase) of chicken liver. The cosubstrates were synthesized by solid-phase synthesis, reduced catalytically, and formylated; a purified enzyme preparation was used and assayed spectrophotometrically following the .DELTA.OD [optical density] at 298 nm resulting from conversion of the formylated folate to the free tetrahydro form. Km values and Vm values determined at saturating concentrations of AICAR and at 25 and 150 mM KCl were used to calculate the relative specificity constants Vm/Km for the N10-formyl-H4PteGlun. At physiologic [K+] (150 mM) they were 1.0, 52, 250, 93, 120 and 59 and at the lower (25 mM) [K+] the relative specificity constants were 1.0, 64, 78, 34, 48 and 37 for n = 1, 3, 4, 5, 6 and 7, respectively. The poly-.gamma.-glutamates are clearly the preferred cosubstrates, particularly when tested at physiologic [K+]. The maximal relative specificity constant observed with N10-formyl-H4PteGlu4 supports the hypothesis that regulation of certain pathways of 1-carbon metabolism may operate via alterations of the poly-.gamma.-glutamyl chain length. No inhibition by the unnatural (d) isomers of the N10-formyl-H4PteGluN was observed.