Studies on the absorption and metabolism of folic acid

Abstract

Folic acid absorption was studied in anesthetized dogs by determining the amount and chemical nature of folate in venous blood emerging from isolated intestinal segments containing free folic acid and/or pteroylpolyglutamates of a known chain length. Chromatographically pure test materials placed in the lumen were prepared by unambiguous solid phase synthetic methods. This synthetic procedure not only yields compounds of known structure, it also provides a means by which glutamic acid residues at any given position in the gamma glutamyl chain can be made radioactive. For example, teropterin (pteroyltriglutamate) was synthesized in such a way that ¹⁴C was present only in the middle glutamic acid unit. Suitable placement of label permitted assessment of the extent of peptide cleavage. The action of plasma conjugase was inhibited by copper chloride. Plasma samples were analyzed by Lactobacillus casei and Streptococcus faecalis assay, by column chromatography, and by quantitative measurement of pteridine-bound radioactivity. It was observed that biologically active folate appeared in the mesenteric vein with either pteroylmono-, di-, tri-, penta-, or heptaglutamate in the lumen. Generally speaking the absorption rate appeared to be inversely related to the length of the gamma glutamyl side chain. Roughly twice as much folic acid appeared in the circulation from ³H-labeled pteroylmonoglutamate as from ¹⁴C-labeled pteroylpentaglutamate when equimolar amounts of each were placed simultaneously in a single intestinal segment. Pteroylmonoglutamate appeared to be the predominant form entering the blood from each of the precursors tested. However, evidence was obtained that pteroyldiglutamate may enter the mesenteric vein soon after placing pteroyldi-, or triglutamate in the lumen, but not with the higher polyglutamates. Comparison of radioactivity and biological activity patterns suggests little conversion, if any, to reduced or methylated forms during the first 30 min of passage through the intestinal mucosa. We conclude that both pteroylmonoglutamates and pteroyldiglutamates may across the intestinal mucosa of the dog, and that reduction and methylation are not essential to the absorption process.