Regulation of rat hepatic lipase by the composition of monomolecular films of lipid.
The regulation of hepatic lipase (HL) by the lipid composition of monomolecular substrate films was examined using a monolayer technique at constant surface pressure. HL-catalyzed hydrolysis of triacylglycerol, a poor substrate for HL in pure monomolecular films, was activated by diradylglycerol and its phosphorylated derivatives in mixed films containing 10 mol % triacylglycerol. When triacylglycerol was progressively diluted with dialkylglycerol, triacylglycerol hydrolysis by HL was maximal between 90 and 98 mol % dialkylglycerol. The best activators, dialkylphosphatidic acid and dialkylphosphatidylethanolamine, increased triacylglycerol hydrolysis 13-14-fold, and the enhancement of HL-catalyzed triacylglycerol hydrolysis by the activator lipids was inversely related to the average mean molecular area of the mixed films. The hydrolysis of 5 mol % triacylglycerol in mixed films that also contained phosphatidylcholine and 0-20 mol % cholesterol was inhibited approximately 80% when the concentration of cholesterol was 10-13 mol %. Interestingly, between 15 and 17 mol % cholesterol the hydrolysis rate was restored to about 50% of the uninhibited rate, but at 20 mol % cholesterol this value decreased back to 80% inhibition of hydrolysis. The hydrolysis of phosphatidylethanolamine in mixed films with 0-20 mol % cholesterol decreased approximately 30% in films containing 10-12 mol % cholesterol. However, at 15 mol % cholesterol the hydrolysis rate was restored to the same level observed for a pure phosphatidylethanolamine film. This enhancement of HL activity occurred at about the same cholesterol concentration as the restoration of triacylglycerol hydrolysis observed for the triacylglycerol/phosphatidylcholine/cholesterol films.(ABSTRACT TRUNCATED AT 250 WORDS)
Wilcox
R W
RW
Department of Medicine, Bowman Gray School of Medicine, Wake Forest University, Winston-Salem, North Carolina 27157.
Thuren
T
T
Sisson
P
P
Schmitt
J D
JD
Kennedy
M
M
Waite
M
M
D000818:Animals; D002784:Cholesterol / Q000032:analysis; D004789:Enzyme Activation; D006868:Hydrolysis; D007700:Kinetics; D008049:Lipase / Q000378:metabolism*; D050356:Lipid Metabolism; D008055:Lipids / Q000032:analysis*; D008099:Liver / Q000201:enzymology*; D010713:Phosphatidylcholines / Q000032:analysis; D010714:Phosphatidylethanolamines / Q000032:analysis / Q000378:metabolism; D011312:Pressure; D051381:Rats; D013499:Surface Properties; D014280:Triglycerides / Q000032:analysis / Q000378:metabolism
D008055:Lipids; D010713:Phosphatidylcholines; D010714:Phosphatidylethanolamines; D014280:Triglycerides; D002784:Cholesterol; D008049:Lipase