Intestinal hydrolysis of pyridoxal 5'-phosphate in vitro and in vivo in the rat. Effect of amino acids and oligopeptides.

The first step in the intestinal absorption of phosphorylated forms of vitamin B6 is intraluminal hydrolysis mediated by alkaline phosphatase. The present studies were performed to evaluate the effect of amino acids and oligopeptides, the products of protein digestion, on the hydrolysis of pyridoxal 5'-phosphate. Models utilized rats and included a cell-free in vitro system and the in vivo, single-pass, perfused jejunal segment. In vitro all amino acids and oligopeptides tested significantly inhibited pyridoxal 5'-phosphate decay (hydrolysis). The degree of inhibition of decay was dependent on the particular compound used, the concentration of that compound, and the pH of the medium. Similar effects for L-lysine concentration and perfusate pH were demonstrated in perfused segments in vivo; by contrast, L-lysine had no effect on pyridoxine uptake. These studies demonstrate that intraluminal hydrolysis of phosphorylated vitamin B6 may be modulated by yet other intraluminal constituents and conditions.