The interconversion of serine and glycine: preparation and properties of catalytic derivatives of pteroylglutamic acid

Abstract

Pteroylglutamic acid (PGA), when hydrogenated in the presence of platinum oxide, takes up 2 molecules of hydrogen in formic acid, acetic acid or neutral aqueous solution. In dilute alkali only one mole of hydrogen reacts rapidly but a further mole is taken up on prolonged shaking. Hydrogenation in formic acid or neutral aqueous solution is more rapid than in the other solvents. Paper chromatograms of the hydrogenation products developed with 0.1 [image] K2HPO4 in an atmosphere of N2 show a number of fluorescing spots. The number and the RF values of these spots depend on the solvent employed for hydrogenation. DihydroPGA formed in dilute alkali showed only one main fluorescent spot on chromatography. DihydroPGA and tetrahydroPGA preparations were all active in stimulating serine synthesis by preparations of rabbit-liver enzyme, irrespective of the solvent employed for hydrogenation. Solutions of dihydroPGA and tetrahydroPGA lost their coenzyme activity for serine synthesis in 30-40 minutes when vigorously shaken in air. This activity could only partly be stored by rehydrogenation, and evidence indicates a considerable loss of the p-aminobenzoylglutamic acid side chain, especially when the solutions are allowed to stand some days. A number of brightly fluorescing compounds, probably simple pteridines, are ultimately formed by oxidative degradation of hydrogenated PGA in air. Among these, xanthopterin and 2-amino-4-hydroxy-6-methylpteridine were identified. PGA accounts for only a small fraction of the oxidation products. No new stable pteridine could be detected by paper chromatography in solutions containing formaldehyde and dihydroPGA or tetrahydroPGA. Incubation of dihydroPGA or tetrahydroPGA with formaldehyde affords slight protection against loss of coenzyme activity during aeration; resulting coenzyme is not stable to oxidation. DihydroPGA retained most coenzyme activity during aeration when treated with 1.5 molecular proportions of formaldehyde, and tetrahydroPGA retained most when treated with 3 molecular proportions. DihydroPGA may form mono-(hydroxymethyl)dihydroPGA as an intermediate in serine synthesis whereas tetrahydroPGA forms bis(hydroxy-methyl)tetrahydroPGA.