Vitamin B-6 metabolic enzymes in blood and placenta of pregnant mice.

Plasma pyridoxal 5'-phosphate (PLP) concentrations decrease 50% in pregnant mice and erythrocyte PLP levels increase threefold over nonpregnant levels. These studies were designed to determine whether changes in the enzymes involved in synthesis and degradation of PLP in blood are altered during pregnancy. We measured net synthesis of PLP in erythrocytes and the activity of enzymes involved in the regulation of plasma and erythrocyte PLP concentration: erythrocyte pyridoxal kinase (PLK) and neutral phosphatase, and plasma and tissue alkaline phosphatase (ALP). Net synthesis of PLP and activities of erythrocyte PLK and neutral phosphatase in erythrocytes remained unchanged during pregnancy. We were unable to detect any dephosphorylation of PLP in erythrocytes of pregnant or nonpregnant mice. Mouse erythrocytes were devoid of ALP activity; neutral phosphatase was inactive with PLP and PLP was an uncompetitive inhibitor of the enzyme. Plasma ALP activity decreased 50% in the pregnant mice; therefore, it likely does not participate in the reduction of plasma PLP levels during pregnancy. Placenta had high levels of PLP-phosphatase activity (ALP) and, if it is active as an ectoenzyme in this tissue as it is in others, it may be the most important mediator of plasma PLP levels in pregnancy.