Title: Vitamin B(6) Metabolism in Human Red Cells: Limitations in Cofactor Activities of Pyridoxal and Pyridoxal 5-Phosphate
Abstract: The relative cofactor activities of the B(6) vitamers in human red cells were assessed by measuring their ability to activate or inhibit the endogenous asparate aminotransferase apoenzyme (apoAST) in undiluted lysates. In the presence of 25 mmol/1 ATP, incubation with 0.15 mmol/1 pyridoxamine (PM) rapidly increased aspartate aminotransferase (AST) levels, reflecting net generation of pyridoxamine 5-phosphate (PMP) and/or pyridoxal 5-phosphate (PLP). Similarly, incubation with 0.15 mmol/1 pyridoxine (PN) inhibited the subsequent activation of apoAST by PLP, a finding consistent with pyridoxine 5-phosphate (PNP) formation. In contrast, incubation with up to 0.6 mmol/1 pyridoxal (PL) neither increased AST activity nor modified the effects of PM and PN on apoAST. Under these conditions, binding of PL to hemoglobin (Hgb) was suggested by an increase in Hgb-oxygen affinity (lower p50 value). The addition of 0.2 mol/1 phosphate or of isoniazid and semicarbazide at concentrations equal to that of PL decreased the interaction of PL with Hgb as reflected by a return of Hgb-oxygen affinities towards control values. In the presence of these agents, PL also inhibited the apparent phosphorylation of PM and PN. Moreover, in the presence of phosphate, incubation with PL led to net activation of apoAST. Similarly, PLP also interacted with Hgb as reflected by an increase in p50 values, and this interaction was decreased by phosphate, isoniazid and semicarbazide. Although PLP was more effective than PMP in activating red cell apoAST in dilute assay systems, this situation was reversed in undiluted lysates. Thus, in the human red cell, (1) both the formation of PLP from PL and the access of PLP to B(6)-dependent apoenzymes are relatively limited, and (2) the nonspecific binding of PL and PLP to Hgb and other proteins may account for the limitations in the net cofactor activities of these aldehydic B(6) vitamers.
Publication Year: 1982
Publication Date: 1982-01-01
Language: en
Type: article
Indexed In: ['crossref', 'pubmed']
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Cited By Count: 5
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