Pyridoxal-5'-phosphate and pyridoxal biokinetics in aging Wistar rats.

Biokinetic parameters of plasma pyridoxal-5'-phosphate (PLP) and pyridoxal (PL) disposition were studied in male Wistar rats aged 8 and 27 months kept from weaning on a purified diet containing 250 g casein and 6 mg pyridoxine.HCl per kg. Baseline plasma PLP concentration was lower in the older animals (514 +/- 56 nmol/L for young and 317 +/- 124 nmol/L for old animals), whereas baseline plasma PL concentration did not differ between age groups (average 235 nmol/L for both young and old animals). We hypothesized lower baseline plasma PLP in the older animals was caused by an increased PLP elimination rate, a decreased PLP synthesis rate, or a combination of these processes. Observations from earlier in vitro experiments suggest age-related changes occur in vitamin B-6 metabolizing enzyme activities. In the in vivo experiments described here no age-related difference in plasma PLP elimination rate nor in plasma PLP synthesis rate was observed to explain the observed decrease in plasma PLP concentration with age.