Characterization and subcellular localization of lipase activities in rat liver cell. Comparison with phospholipase A

Abstract

Summary u 1. – This study deals with the localization, characterization and properties of lipases present in rat liver subcellular fractions. 2. – No triglyceride lipase activity was detected in purified mitochondrial preparations. 3. – Lysosomes, isolated as tritosomes, contain a triglyceride-lipase very active against long chain fatty acid triglycerides. It is a soluble enzyme and has a pH optimum at 4.5. 4. – A triglyceride-lipase activity is also found in microsomes and plasma membranes; it is membrane-bound. In microsomes the pHopt is 8 and the apparent KM 27 mM (triolein emulsified in 5 p. cent arabic gum). The pHopt is 9–9.5 and the apparent KM 9 mM for plasma membranes (same substrate). 5. – A very active monoglyceride-lipase has been detected in smooth and rough endoplasmic reticulum membranes (pHopt 9). Monoglyceride-lipase activity with a pHopt of 9 is also present in plasma membrane. 6. – An attempt was made to determine whether the lipase and phospholipase A1 activities, which are both present in endoplasmic reticulum and in plasma membranes, are due to the same enzyme. The response to heat treatment, the different extractibility by salts or detergents, the differential inactivation of phospholipase A by removal or alteration of lipid membrane components suggest but do not prove that the triglyceride-lipase, the monoglyceride-lipase and the phospholipase A1 are different protein entities.