SummaryDigestion of dietary fat starts in the stomach, where lingual lipase hydrolyzes triglycerides to free fatty acids and partial glycerides at pH 3.0-6.0. Lingual lipase is secreted continuously from lingual serous glands and accumulates in the stomach between meals, when gastric pH is <3.0. We have, therefore, examined the resistance of lingual lipase to low pH and its possible protection by dietary components present in the stomach contents. Partially purified rat lingual lipase (7-15 µg enzyme protein) was preincubated at 37°C for 10-60 min at pH 1.0-6.0 before incubation for assay of lipolytic activity, hydrolysis of tri- [3H]olein at pH 5.4. The data show that partially purified rat lingual lipase preparations are stable at 37°C in the pH range of 2.5-6.0. Enzyme activity, however, is rapidly and irreversibly lost during preincubation at pH 1.0-2.4 for 10-30 min. Protein (gelatin 1% or albumin 1% or 2.5%) cannot prevent the inactivation of lingual lipase at low pH. The large molecular species (molecular weight >500,000) of lingual lipase (thought to be an aggregate of enzyme with lipids) is slightly more resistant to inactivation than the 46,000 dalton preparation, suggesting that lipids might protect the enzyme from inactivation. Indeed, about 60% of the initial lipase activity is preserved during incubation at pH 2.0 in the presence of 50 mM lecithin or 10 mM triolein. The data indicate that triglycerides which are hydrolyzed by this enzyme as well as phospholipids that are not hydrolyzed can prevent the inactivation of the enzyme. Lingual lipase is resistant to peptic digestion, even at high pepsin concentrations. Because of the special importance of lingual lipase in neonatal fat digestion, we compared the stability of the lipase in gastric aspirates of newborn infants to that of rat lingual lipase. The lipase in human gastric aspirates is more resistant to inactivation than the rat enzyme. There are however individual variations in enzyme stability: from preservation of 100% of original activity to the loss of 80% of activity during incubation for 30 min at pH 1.6. The data suggest that substances within the newborn infant's stomach stabilize the enzyme at low pH. We suggest that the extensive intragastric hydrolysis of dietary fat (in formula or breast milk) is related to the stability of the lipase in the stomach of newborn infants.