Hydrolysis of membrane phospholipids by phospholipases of rat liver lysosomes.
(1) The hydrolysis of (32)P- or myo-[2-(3)H]inositol-labelled rat liver microsomal phospholipids by rat liver lysosomal enzymes has been studied. (2) The relative rates of hydrolysis of phospholipids at pH4.5 are: sphingomyelin>phosphatidylethanolamine>phosphatidylcholine> phosphatidylinositol. (3) The predominant products of phosphatidylcholine and phosphatidylethanolamine hydrolysis are their corresponding lyso-compounds, indicating a slow rate of total deacylation. (4) Ca(2+) inhibits the hydrolysis of all phospholipids, though only appreciably at high (>5mm) concentration. The hydrolysis of sphingomyelin is considerably less sensitive to Ca(2+) than that of glycerophospholipids. (5) Analysis of the water-soluble products of phosphatidylinositol hydrolysis (by using myo-[(3)H]inositol-labelled microsomal fraction as a substrate) produced evidence that more than 95% of the product is phosphoinositol, which was derived by direct cleavage from phosphatidylinositol, rather than by hydrolysis of glycerophosphoinositol. (6) This production of phosphoinositol, allied with negligible lysophosphatidylinositol formation and a detectable accumulation of diacylglycerol, indicates that lysosomes hydrolyse membrane phosphatidylinositol almost exclusively in a phospholipase C-like manner. (7) Comparisons are drawn between the hydrolysis by lysosomal enzymes of membrane substrates and that of pure phospholipid substrates, and also the possible role of phosphatidylinositol-specific lysosomal phospholipase C in cellular phosphatidylinositol catabolism is discussed.
D000818:Animals; D002118:Calcium / Q000494:pharmacology; D005230:Fatty Acids, Nonesterified / Q000378:metabolism; D005989:Glycerides / Q000096:biosynthesis; D006868:Hydrolysis; D007295:Inositol Phosphates / Q000378:metabolism; D008099:Liver / Q000201:enzymology*; D008244:Lysophosphatidylcholines / Q000096:biosynthesis; D008247:Lysosomes / Q000201:enzymology*; D008563:Membrane Lipids / Q000378:metabolism*; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism; D010716:Phosphatidylinositols / Q000378:metabolism; D010740:Phospholipases / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism*; D051381:Rats
D005230:Fatty Acids, Nonesterified; D005989:Glycerides; D007295:Inositol Phosphates; D008244:Lysophosphatidylcholines; D008563:Membrane Lipids; D010716:Phosphatidylinositols; D010743:Phospholipids; D010740:Phospholipases; D002118:Calcium