Rat lingual lipase: effect of proteases, bile, and pH on enzyme stability

In addition to initiating fat digestion in the stomach, lingual lipase may play a significant digestive role in the upper small intestine. By in vitro incubation techniques, the stability of rat lingual lipase at various physiological pH values, as well as the effects of pure proteases, rat gastric juice, bile, pancreatic juice, and mixed duodenal contents, on enzyme activity was explored. There were no changes in base-line activity of porcine pepsin, bovine carboxypeptidase-treated lipase, or heat-denatured proteases compared with controls after incubation at pH 2-6 at 37 degrees C for up to 1 h. In contrast, porcine trypsin-treated lipase demonstrated a significant loss from base-line activity to 59 +/- 12% (mean +/- SE) at pH 4, 34 +/- 11% at pH 6, and 41 +/- 4% at pH 8, and bovine chymotrypsin caused a loss in lipase activity to 11 +/- 7% at pH 8. Rat gastric juice containing 5,000 U pepsin reduced lipase activity to 17 +/- 5% of initial activity at pH 2 and to 45 +/- 3% at pH 4. Rat bile alone diminished activity only 35%, but rat pancreatic juice or mixed duodenal juice reduced lingual lipase activity to 1-12% of initial activity after 60 min at pH 6. Lingual lipase is particularly important in fat digestion in the stomach; however, its role in quantitative fat digestion under small intestinal conditions may be limited.