Studies on the Metabolism of Labeled Pyridoxine in Man

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Abstract
Tritium-labeled pyridoxine was administered orally to one subject and intravenously to two subjects. The urinary excretion of isotope was followed daily for up to twenty-five days, and the specific radioactivity of the urinary 4-pyridoxic acid was determined. During the first days after administration of the pyridoxine, 15 to 20 per cent of the administered isotope was excreted in the urine; the remainder that was eliminated had a half-life of eighteen to thirty-eight days. A simple model for estimating the over-all metabolism of pyridoxine in human subjects is proposed, which consists of a small compartment with a rapid turnover rate and a larger storage compartment. The rate of elimination of vitamin B6 from the body reservoir was estimated to be 2 to 3 per cent per day and the total turnover rate of the vitamin to be 2.2 to 4.4 per cent per day. Approximate values were calculated for the body content of the vitamin, 40 to 150 mg., and for the daily elimination, 1.7 to 3.6 mg. per day. The isotope-containing compounds in the urine were fractionated by ion exchange chromatography and it was found that 4-pyridoxic acid accounted for 20 to 40 per cent of the amount of isotope eliminated.