Bicarbonate-stimulated ATPase activity of bovine liver alkaline phosphatase

A commercial preparation of bovine hepatic alkaline phosphatase was found to have a Mg2+-stimulated ATPase activity. The pH optimum was 8.5, the Km for ATP was 4.2 X 10(-5) M and the Vmax was 88.3 mumol Pi . h-1 . mg-1. HCO3- had a stimulatory effect on Mg2+ -ATPase activity. Other anions had no effect or an inhibitory effect while Na+, K+ and ouabain had no effect. Purification of the commercial preparation by gel filtration and affinity chromatography yielded a fraction with alkaline phosphatase and (Mg2+ + HCO3-)ATPase activities that had been enriched respectively 27-fold and 23-fold; both activities were inhibited by levamisole (93.1% and 93.8%, respectively) and the purified fraction was found to be a single protein on sodium dodecyl sulfate polyacrylamide gel electrophoresis. These results suggest that alkaline phosphatase and (Mg2+ + HCO3-)ATPase may be properties of the same liver protein that might be involved in biliary HCO3- transport and bile secretion.