Activation of Human Pancreatic Lipase Activity by Calcium and Bile Salts

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Abstract
We examined the effects of calcium, bile salts and colipase, a protein cofactor, on pancreatic lipase purified from human duodenal juice. It was found that 10% gum acacia, used as a stabilizer of triglyceride emulsions, was contaminated with a high concentration of calcium—between 14 and 20 m m . Olive oil stabilized with gelatin was used as a substrate to decrease the calcium content in the reaction mixture. The activity of duodenal juice and pancreatic extracts was inhibited by 50% with 10 μM EGTA. Ca 2+ activated pancreatic lipase, irrespective of the presence of bile salts plus colipase. The apparent Ka for Ca 2+ was calculated to be 10 μm and this was not affected by colipase and bile salts. However, maximal activation by Ca 2+ required both bile salts and colipase. Ca 2+ increased Vmax of the reaction and decreased the apparent K m for the substrate in the presence of both colipase and bile salts. However, no binding of 45 Ca to colipase was demonstrated. High concentrations of bile salts inhibited the activity in the presence of either colipase or Ca 2+ , while in the presence of both colipase and Ca 2+ a marked activation by bile salts was observed. Colipase by itself also activated lipase. We conclude from the present experiments that the activity of human pancreatic lipase is regulated by three effectors; Ca 2+ , bile salts and colipase, which may enhance each other's efficiency in the enzyme activation. In addition, we suggest that an activation rather than an inhibition of pancreatic lipase by bile salts is more significant physiologically, because it is well-known that bile salts play an important role in fat absorption in the intestine.