Hepatic Vitamin B6 Metabolism

nM. Thus, pyridloxal-P is the principal B6 vitaimer in plasma. Durinig pyridoxine stupplemiienitation, mealn plasnma levels of the B6 vitaimers anid pyridoxic acidl increasedl to 655±122 aindl 222±55 nM, respectively. The plasma content of pyridoxal-P aind( pyricloxic acidl increased 6-7-fold anid thatt of pyridoxal, 12-fold, but the pyridloxiine level did niot increase. Isolatedlhepatocytes, 1 g/15 ml, were incubated for 2 h with 3.33 ,uM [14C]pyridoxine (6 ,uCi/,umol). At zero time, the cells containied about 35 nmol pyridoxal-P and 25 nmol pyridoxamiine-P. After 2 h incubation, the cellular content of' pyridoxal-P and pyridoxaminie-P dlid niot chIaInge significantly, but the meidiunm containetid 5.9 nmol pyridoxal-P, 0.3 nmol pyridoxamine-P, 7.2 nmol pyridloxal, 26.6 nmol pyridoxine, 0.3 nmol pyridoxaminie, and 7.5 iumol pyridoxic acid. Whereats the specific radlioaetivity of pyricloxal-P, pyridoxal, andl pyricloxic acid in the mediuimi approached that of [14C]pyridoxine, the specific radioactivity of celltular pyridoxal-P andl pyricloxamine-P was only 20% of that of pyricloxine. Thuis, newly syynthesized pyricloxal-P is not freely exchalngeable with endogenous pyridoxal-P, but is preferentiatlly releasedl or degraded to pyri(loxal aindl pyridoxic acidl. The latter B6 compounids are also released. These resuilts suggest that orally ingestedl pyri(loxine is rapidly Receivedffor publication 14 March 1980 anid in revised form 13 Junte 1980. 688 metabolized in liver and its products are released into the circulation in the form of pyridoxal-P, pyridoxal, acnd pyridoxic acid.