A B S T R A C T The plasnai content of B6 vitanmers is governed by, anmong other factors, dietary supply anid metabolic interconversion. This study examines the effect of pyridoxinie supplementation on the plasma content of B6 vitaimers anid pyridoxic acid in man, and the metabolic conversioni aind releatse of B6 compounlds in isolated rat hepatocytes. Six healthy humiian sulbjects were givein 100n)mg pyridoxine-HCI/d orally for 1-3 wk. Before pyridoxine stupplemenetationi, the mean total plasma level of B6 vitamners was 114+9 nM; acnd( pyridoxal-P, pyridoxamine-P, pyridoxal, pyridoxine, aind pyridoxaminie accounted for 54, 3, 11, 27, ani(d 5%, respectively. Plasmna level of pyridoxic acid was 40±7 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. INTRODUCTIONVitamin B6 is metabolized in vivo to multiple vitameric forms and isiultimnately excreted as pyridoxic acid in the urine. Pyridoxal-P anid pyridoxamnine-P, the coenzyme forms, are the maljor B6 comIIpoun(ls in tissues (1), but pyridoxal-P (2) acnd pyridoxal (3) appeatr to be the major form-iis in 10loo0( plasmla. The (quaintitative relationishlip between these B6 vitamers acnd( otlier B6 compounds in plasmia hals not been defined. Such knowle(dge is important to the understacnding of the process of tranlsport of vitaimin B6 aimionig the differenit orgains and(c tissues of the lbody.Reeiently, we have modified an existing method (4) for the separation and ancalysis of the B6 com11poun11dS.By using this method, we have miieasture(d ...