Role of apolipoprotein A-IV in hepatic lipase-catalyzed dolichol acylation and phospholipid hydrolysis.

Hepatic lipase catalyzes the hydrolysis of phospholipids and neutral glycerides as well as transacylation reactions between several of these lipids. We have previously reported that this enzyme also transacylates the sn-I fatty acid of phosphatidylethanolamine to dolichol and that this reaction requires a plasma cofactor. In this study, we have purified the cofactor from the lipoprotein-free fraction of human plasma and present evidence demonstrating that it is identical to apolipoprotein A-IV. The effect of apolipoprotein A-IV on hepatic lipase-catalyzed dolichol acylation and phospholipid hydrolysis was studied in model membranes and compared with the effects of apolipoprotein A-I and E. Apolipoprotein A-IV strongly stimulated dolichol acylation and phosphatidylethanolamine hydrolysis but partly inhibited phosphatidylcholine hydrolysis. Apolipoprotein A-I had only a minor influence on the various activities studied and could not replace apolipoprotein A-IV. Apolipoprotein E stimulated the hydrolysis of both phospholipids but had no effect on dolichol acylation. The effect of apolipoprotein A-IV on hepatic lipase activity was then studied with the gum arabic-stabilized triglyceride emulsion. The apolipoprotein neither stimulated nor inhibited triglyceride hydrolysis in the emulsion. Finally, human high-density lipoprotein-2 and very low-density lipoprotein were also used as substrates. Apolipoprotein A-IV strongly stimulated the hydrolysis of phosphatidylcholine and phosphatidylethanolamine in both lipoproteins, while the hydrolysis of triglycerides was completely inhibited. These results demonstrate that apolipoprotein A-IV is an important cofactor to hepatic lipase affecting both catalytic rates and the substrate specificity of the enzyme. We therefore suggest that apolipoprotein A-IV-rich high-density lipoprotein is the preferred substrate for hepatic lipase.