METABOLISM OF VITAMIN-B-6 BY HUMAN KIDNEY

A continuous intravenous pyridoxine-infusion (dose = 100 mg PN.HCl; ta u = 6 h) was given to 10 healthy male volunteers (aged between 23 and 32). Concentration-time curves of B-6-metabolites in urine and blood p lasma were observed over a period of 240 hours. From these curves biok inetic parameters were calculated. Renal clearances (ml/min/1.73 m(2)) were 257.5 +/- 92.1 (pyridoxine), 249.8 +/- 48.2 (4-pyridoxic acid), and 20.1 +/- 6.0 (pyridoxal). Rate constants of elimination (h(-1)) ob tained by non-linear regression of the excretion curves were 1.231 +/- 0.877 (pyridoxine), 0.314 +/- 0.118 (k(alpha)) and 0.048 +/- 0.070 (k (beta)) (4-pyridoxic acid), and 0.653 +/- 0.360 (pyridoxal). The main part of the infused pyridoxine was eliminated in the form of metabolit es (63.7 +/- 6.4% for 4-pyridoxic acid, 3.3 +/- 0.9% for pyridoxal); o nly 6.7 +/- 1.8% was excreted unchanged. The proportion of 4-pyridoxic acid within each miction increased from 53.0 to 100.0%. Our data indi cate a very effective metabolism of the physiologically inactive pyrid oxine. An enzyme induction suggested by the increasing proportion of 4 -pyridoxic acid cannot be excluded. Large doses of vitamin B-6 are exc reted rapidly via the kidney, with the excretion of pyridoxine and 4-p yridoxic acid being enhanced by tubular secretion.