Thiamin Metabolism in Man

Abstract

The metabolism of 2-¹⁴C-thiazole-labeled thiamin and 2-¹⁴C-pyrimidine-labeled thiamin was studied in four healthy young adult men. The orally administered labeled thiamin was almost completely absorbed with no measurable amount of ¹⁴CO₂ detected in the respiratory air. The half excretion time of the ingested label occurred within 9½-l8½ days and appeared to be related to the weight of the subject and the daily dietary intake of thiamin. The urinary radioactive metabolites of thiamin were fractionated by Amberlite CG-50 column chromatography. Four radioactive peaks were obtained from the urine of three subjects who had ingested 2-¹⁴C-thiazole-labeled thiamin and three peaks from the urine of a subject who had received 2-¹⁴C-pyrimidine-labeled thiamin. Each peak was tested separately for its biological activity for L. viridescens. Approximately 50% of the thiamin activity present in the urine was thus found due to metabolites other than that of free thiamin. The urinary radioactive metabolites resulting from both types of labeled thiamin administration were further separated by thin-layer chromatography and autoradiography of the radioactive peaks obtained from the column chromatography. The use of 2-¹⁴C-pyrimidine-labeled thiamin resulted in 10 metabolites, of which four were major, whereas 2-¹⁴C-thiazole-labeled thiamin produced 18 metabolites, of which six were major. One of the major urinary metabolites of 2-¹⁴C-thiazole-labeled thiamin was positively identified as 4-methyl-thiazole-5-acetic acid. Also the thiazole moiety of thiamin and human diphosphate were tentatively identified as two minor catabolic products of thiamin. One of the major urinary metabolites of thiamin, which contained both the pyrimidine and the thiazole moieties, appeared to be a conjugation of thiamin or a derivative with a peptide moiety.