Lysosomal metabolism of lipids.

Mammalian cells are very active in lipid metabolism, constantly acquiring new lipid by synthesis and by endocytosis of exogenous materials, and constantly degrading lipid by a combination of lipolytic and oxidative processes. The lysosome is a critical component in the degradative arm of this system and is primarily responsible for the lipolysis of both exogenous lipids (e.g., the lipid acquired by endocytosis of lipoproteins) and endogenous membrane lipids that are delivered to this organelle as a result of endocytosis and lysosome biogenesis. The lipolysis of complex lipids (e.g., glycerophospholipids, cholesteryl esters, etc.) in lysosomes yields relatively simple lipid and nonlipid products, which subsequently diffuse or undergo transport through the lysosomal limiting membrane to become available for oxidation, efflux, or biosynthetic reutilization elsewhere in the cell. The degradation of lipids within lysosomes is catalyzed by several specific lipases, which have acidic pH optima and which are concentrated in this organelle. There are several lysosomal storage diseases attributable to the inherited absence of individual acid lipases. In addition, lysosomes play a central role in the turnover of plasma lipoproteins and thereby participate in processes that paradoxically tend to prevent and promote atherosclerosis, a major health problem in the developed Western nations.