Intestinal hydrolysis of pyridoxal 5′-phosphate in vitro and in vivo in the rat: Effect of protein binding and pH

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Phosphatase-mediated hydrolysis is the first step in the intestinal absorption of pyridoxal 5′-phosphate (PLP). Studies presented here evaluated the effects of PLP-protein binding and pH on intestinal hydrolysis in the rat. Models included in vitro PLP decay and in vivo PLP disappearance from perfused jejunal segments. Pyridoxal 5′-phosphate binding to albumin was pH-, PLP concentration-, and albumin concentration-dependent. Binding occurred at higher pH levels (5–7.4) and markedly inhibited hydrolysis of 2 μM PLP both in vitro and in vivo. At low pH (3–4), alkaline phosphatase was still active with PLP as substrate; binding of PLP to albumin was negligible; and PLP hydrolysis was unaffected by even a high concentration of albumin. The pH required to prevent binding and thus in turn prevent inhibition of hydrolysis was a function of albumin concentration: the higher the concentration, the lower the pH necessary. The present studies suggest an important role for the low pH values resulting from gastric acid secretion in the normal absorption of dietary PLP, and raise questions about the implications of widespread use of acid-lowering therapeutic modalities on vitamin B6 nutritional status.