ROLE OF APOLIPOPROTEIN A-IV IN HEPATIC LIPASE-CATALYZED DOLICHOL ACYLATION AND PHOSPHOLIPID HYDROLYSIS

Hepatic lipase catalyzes the hydrolysis of phospholipids and neutral g lycerides as well as transacylation reactions between several of these lipids. We have previously reported that this enzyme also transacylat es the sn-1 fatty acid of phosphatidylethanolamine to dolichol and tha t this reaction requires a plasma cofactor. In this study, we have pur ified the cofactor from the lipoprotein-free fraction of human plasma and present evidence demonstrating that it is identical to apolipoprot ein A-IV. The effect of apolipoprotein A-IV on hepatic lipase-catalyze d dolichol acylation and phospholipid hydrolysis was studied in model membranes and compared with the effects of apolipoprotein A-I and E. A polipoprotein A-IV strongly stimulated dolichol acylation and phosphat idylethanolamine hydrolysis but partly inhibited phosphatidylcholine h ydrolysis. Apolipoprotein A-I had only a minor influence on the variou s activities studied and could not replace apolipoprotein A-IV. Apolip oprotein E stimulated the hydrolysis of both phospholipids but had no effect on dolichol acylation. The effect of apolipoprotein A-IV on hep atic lipase activity was then studied with the gum arabic-stabilized t riglyceride emulsion. The apolipoprotein neither stimulated nor inhibi ted triglyceride hydrolysis in the emulsion. Finally, human high-densi ty lipoprotein-2 and very low-density lipoprotein were also used as su bstrates. Apolipoprotein A-IV strongly stimulated the hydrolysis of ph osphatidylcholine and phosphatidylethanolamine in both lipoproteins, w hile the hydrolysis of triglycerides was completely inhibited. These r esults demonstrate that apolipoprotein A-IV is an important cofactor t o hepatic lipase affecting both catalytic rates and the substrate spec ificity of the enzyme. We therefore suggest that apolipoprotein A-IV-r ich high-density lipoprotein is the preferred substrate for hepatic li pase.