Metabolism of vitamin B 6 by human kidney

Abstract

A continuous intravenous pyridoxine-infusion (dose = 100 mg PN·HCl τ = 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 plasma were observed over a period of 240 hours. From these curves biokinetic 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) obtained by non-linear regression of the excretion curves were 1.231 ± 0.877 (pyridoxine), 0.314 ± 0.118 (k α) and 0.048 ± 0.070 (k β) (4-pyridoxic acid), and 0.653 ± 0.360 (pyridoxal). The main part of the infused pyridoxine was eliminated in the form of metabolites (63.7 ± 6.4% for 4-pyridoxic acid, 3.3 ± 0.9% for pyridoxal); only 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 indicate a very effective metabolism of the physiologically inactive pyridoxine. An enzyme induction suggested by the increasing proportion of 4-pyridoxic acid cannot be excluded. Large doses of vitamin B 6 are excreted rapidly via the kidney, with the excretion of pyridoxine and 4-pyridoxic acid being enhanced by tubular secretion.