Pancreatic lipase activity as influenced by unconjugated bile acids and pH, measured in vitro and in vivo

The relation between pancreatic lipase activity, unconjugated bile acids and pH was studied in vitro and in vivo. Lipase activity was assayed in vitro using automatic titration, where the fatty acids liberated from the hydrolysis of glycerol tributyrate (GTB) were measured. The lipase activity was determined at different ratios of conjugated to unconjugated bile acids (100:0, 75:25, 50:50, 25:75, 0:100) in response to pH 6.6, 6.8, 7.0 and 7.5. The in vivo study involved 96 one-day-old male broiler chickens. The chickens were assigned randomly, in pens of six animals, into two dietary treatments (8 replicate blocks), composing a non-supplemented diet (A-) and a diet supplemented (A+) with avilamycin (10 mg/kg feed) and salinomycin (40 mg/kg feed). After 35 days, the chickens were killed and content of the proximal part of the small intestine was collected and analyzed for bacterial counts, pH, bile acid concentration, and lipase activity. Evidence for a significant pH-dependent inhibition of lipase activity by unconjugated bile acids was provided in vitro and confirmed in vivo. Due to a reduction in nutrient fermentation, the pH in the small intestine of antibiotic-fed chickens was significantly higher than in chickens fed the non-supplemented diet. The high pH in the small intestine of chickens fed the A+diet was accompanied by a significant increase in lipase activity, and coincided with a significantly lower concentration of unconjugated bile acids and a higher ratio of conjugated to unconjugated bile acids. This study emphasizes the important influence of unconjugated bile acids on lipase activity at physiological pH-values.