Abstract
In order to elucidate the physiological function of intestinal alkaline phosphatase, the characteristics of alkaline phosphatases from rat and human small intestine were compared under optimal and physiological pHs. The Km values of these enzymes towards p-nitrophenylphosphate at the physiological pH were lower by two orders of magnitude than those at the optimal pH. At the physiological pH, phosphate, arsenate and vanadate competitively inhibited alkaline phosphatase activity, as they did at the optimal pHs, and the Ki values of these inhibitors at the physiological pH were also lower by two orders of magnitude than those at the optimal pHs. The effects of various inhibitors and antiserum to rat intestinal alkaline phosphatase on the transport of phosphate into everted rat intestine were investigated. The results obtained from the present study indicate that a phosphate transport system operating at physiological pH exists in the upper part of the small intestine where the alkaline phosphatase is maximally concentrated. It was also found that the phosphate transport was affected by various inhibitors and antiserum to rat intestinal alkaline phosphatase, but L-homoarginine and ouabain had no effect. From the above findings, it is suggested that alkaline phosphatase may function not only as a hydrolytic enzyme of phosphomonoesters but also as a phosphate transporter in the physiological state.
