Isolation and characterization of four lipolytic preparations from rat skeletal muscle

Four different lipolytic preparations have been isolated from rat skeletal muscle. Two of these, provisionally designated monopalmitin lipase (MPL) and monomyristin lipase (MML), are associated with insoluble cellular particulate fractions. The other two enzymes, provisionally designated tricaproin lipase (TCL) and monolaurin lipase (MLL), are found in the high-speed supernatant fraction. Taken as a group, these enzymes are capable of hydrolyzing short-chain triglycerides (acyl moieties of C(3) to C(8)) and monoglycerides of lauric, myristic, palmitic, stearic, and oleic acids. All of these enzymes have a serine residue at or near the catalytic site as they are strongly inhibited by diisopropyl fluorophosphate. The two particulate preparations contain a sulfhydryl group and are sensitive to p-chloromercuribenzoate and N-ethylmaleimide, while the soluble preparations are not. The MLL, MML, and MPL preparations all have alkaline pH optima, while the TCL preparation has an acidic optimum. Buffer type is important: some buffer compounds completely inhibited one or more preparations. Of the soluble enzymes, MLL withstood heating to 60 degrees C, while TCL is completely inactivated at this temperature. Of the particulate preparations, only MML was stable to lyophilization. It is concluded that there are at least four lipolytic enzymes in rat skeletal muscle. The possible significance of the presence of these enzymes in muscle is discussed.