Digestive Activity of Lysosomes

Abstract

Abstract The ability of rat liver lysosomes to digest various lipids has been investigated. The lysosomes were isolated from the livers of rats treated with Triton WR-1339 and were essentially free of other cell components. They were able to deacylate extensively phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, lysophosphatidylcholine, lysophosphatidylethanolamine, phosphatidic acid, cardiolipin, tripalmitin, 1,2-dipalmitin, 1,3-dipalmitin, and 1-monopalmitin, when incubated with these substrates at 37° in 0.1 m acetate buffer, pH 4.3 to 4.6. With most substrates, hydrolysis occurred without the addition of a detergent. Exceptions were tripalmitin, which was not attacked in the absence of detergent, but was deacylated vigorously in the presence of 5% Triton X-100, and cardiolipin, which required addition of 0.1% Triton X-100 for hydrolysis in acetate buffer, but was attacked in citrate buffer without added detergent. The hydrolysis of diglycerides was stimulated by 5% Triton X-100. On the other hand, this detergent inhibited the deacylation of phosphatidylethanolamine and of monopalmitin. Sphingomyelin was not deacylated, but was hydrolyzed to ceramide and phosphorylcholine. Hydrolysis of ceramide could not be established, either with lysosomes or with whole liver preparations, under a variety of conditions. The phosphodiesters arising from the digestion of phospholipids proved to be resistant to lysosomal hydrolysis, both at acid and alkaline pH. An alkaline phosphodiesterase acting on glycerophosphorylcholine is present in whole liver. Part of this activity is located in the cell sap; the remainder is distributed between the nuclear and microsomal fractions in a manner suggestive of a localization in the plasma membrane. The lysosomes were found to have the ability to dephosphorylate phosphatidic acid, α-glycerophosphate, and, although rather slowly, the phosphomonoesters of choline, ethanolamine, and serine. Extensive digestion of the lipids and proteins of both microsomes and mitochondria occurs upon prolonged incubation of these cell components with purified lysosomes at pH 4.3. RNA appears to be broken down under these conditions. Thus it is clear that lysosomes can accomplish the important digestive functions with which they are credited. However, the manner in which some of the products of this process, especially the phosphodiesters, are cleared from the particles raises a problem of physiological importance.