TRIGLYCERIDE BREAKDOWN IN RAT LIVER DEMONSTRATION OF 3 DIFFERENT LIPASES

Triglyceride hydrolysis by homogenate fractions of rat liver was measured by determining free fatty acids and glycerol after incubation with artificial triolein or tributyrin emulsions. Triolein lipolysis exhibited two pH maxima at pH 5 and 8.5, respectively. Acid lipase was shown by differential centrifugation to be of lysosomal origin and was purified 300-fold from Triton-filled lysosomes. This enzyme could be activated by sonication and by hypotonic treatment like other lysosomal enzymes. Alkaline lipase was found to sediment with the microsomes. In this fraction hydrolysis of tributyrin was more than 6 times faster than lipolysis with triolein as substrate. The ratio of free fatty acids to glycerol released was above 3 at acid pH and below 3 at alkaline pH. Heparin was shown to stimulate triolein hydrolysis with a maximum at pH 7.5. This heparin-sensitive lipase could be localized in a plasma membrane preparation and could only be demonstrated in the presence of Ca2+. The results suggest the existence of three distinct lipases in rat liver with different pH maxima and subcellular localization. Their activities are compared with data reported in the literature, and a possible physiological role of the lipases is discussed. © 1969.