Kinetic evidence for phosphatidylethanolamine and triacylglycerol as preferential substrates for hepatic lipase in HDL subfractions: modulation by changes in the particle surface, or in the lipid core.
Human HDL subfractions, HDL2 (d: 1.085-1.125) and HDL3 (d: 1.125-1.19) labelled with 2-[14C]linoleoylphosphatidylethanolamine and tri-[3H]oleoylglycerol, were incubated with partially purified hepatic triacylglycerol lipase, isolated from human post-heparin plasma. Kinetics of hydrolysis of these two HDL-lipid substrates were followed and were compared to those previously obtained on phosphatidylcholine (G. Simard et al (1989) Biochim. Biophys. Acta 1001, 225-233). (1) The apparent Km obtained for HDL-triacylglycerol was half that for HDL-phosphatidylethanolamine, but the estimated Vmax was higher for the latter. Hence, despite a lower affinity, more molecules of phosphatidylethanolamine than of triacylglycerol were found hydrolysed. A strong correlation was observed between the hepatic lipase activity added and the maximal degradation rates for phosphatidylethanolamine measured in HDL2 and HDL3. (2) A linear relationship was observed in both HDL2 and HDL3 between the respective degradations of the two substrates. The number of phosphatidylethanolamine molecules hydrolysed exceeded that of triacylglycerol by 30% in HDL2 and by 70% in HDL3. HDL2 were 2- and 4-times more reactive than HDL3 for the hydrolysis of phosphatidylethanolamine and triacylglycerol, respectively, taking the Vmax/Km ratio as an indicator of catalytic efficiency. In both HDL subfractions, the calculated Vmax/Km value was 30-50-fold higher for PE and TG than for PC. (3) HDL particles were modified either on their surface by selective enrichment in free cholesterol or in their inner-core by replacement of esterified cholesterol by triacylglycerol in presence of a source of neutral lipid transfer activity. A mild cholesterol enrichment stimulated the phosphatidylethanolamine and triacylglycerol reactivities by 30-60% towards hepatic lipase, whereas increasing the triacylglycerol concentration in HDL was followed by a proportional increase in the amounts of triacylglycerol hydrolysed with no effect on phospholipid degradation.
INSERM Unité 326, Hôpital Purpan, Toulouse France.
C
C
Azéma
P
P
Marques-Vidal
A
A
Lespine
G
G
Simard
H
H
Chap
B
B
Perret
D002784:Cholesterol / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008049:Lipase / Q000097:blood / Q000378:metabolism*; D008075:Lipoproteins, HDL / Q000032:analysis / Q000378:metabolism*; D010713:Phosphatidylcholines / Q000378:metabolism; D010714:Phosphatidylethanolamines / Q000378:metabolism*; D013379:Substrate Specificity; D014280:Triglycerides / Q000378:metabolism*
D008075:Lipoproteins, HDL; D010713:Phosphatidylcholines; D010714:Phosphatidylethanolamines; D014280:Triglycerides; D002784:Cholesterol; D008049:Lipase