Intralysosomal glycerophospholipid catabolism in liver: hydrolysis of amino alcohol-containing phospholipids and their metabolites.

Liver lysosomes were isolated from untreated rats and rats pretreated with Triton WR-1339. Purified lysosomes were also separated into lysosomal matrix and membrane fractions. With freshly prepared and frozen biological material, the lysosomal catabolism of various stereospecifically radiolabeled amino alcohol-containing glycerophospholipids and their potential metabolites was studied. Basically there was no qualitative difference in the formation of phospholipid metabolites in both preparations: after long-term incubation, free fatty acids, lysophospholipids, acyl-free phosphodiesters were detected, and to a far lesser extent, amino alcohol-containing phosphomonoesters and only traces of free amino alcohols. These findings indicate the presence of lysosomal phospholipases A as well as C and lysophospholipase(s), with pH optima of about 4.5, and they clearly exclude phospholipase D activity. Unfractionated lysosomes and their soluble as well as particulate subfractions were not capable of hydrolysing the acyl-free amino alcohol-containing phosphodiesters. These compounds must therefore be considered one of the end products of the intralysosomal catabolism of amino alcohol-containing phosphoglycerides. They are presumably cleared from the lysosomal compartment by an as yet unknown transport system in the lysosomal membrane. In liver, the extralysosomal site of their (Mg(2+)-dependent) hydrolysis seems to be the plasma membrane. By contrast, hydrolysis of glycero-3-phosphate and the amino alcohol-containing phosphomonoesters was catalysed in the lysosomal compartment, with a pH optimum of about 5.0, although at considerably lower rates than that of glycero-2-phosphate, a model substrate for lysosomal acid phosphatase.