Ex vivo measurement of lipoprotein lipase-dependent very low density lipoprotein (VLDL)-triglyceride hydrolysis in human VLDL: An alternative to the postheparin assay of lipoprotein lipase activity?

The plasma lipolysis of triglyceride (TG)-rich lipoproteins is mainly due t o the activity of lipoprotein lipase (LPL). Albeit important for our analys is of certain physiopathological situations, the determination of the magni tude of LPL-dependent lipolysis is not easy to perform. This essentially re sults from the binding of LPL to the luminal surface of vascular endotheliu m. The measurements of the whole putative LPL activity have been achieved a fter injection of heparin, a procedure that releases LPL from endothelium. However, the physiopathological relevance of this postheparin lipolysis ass ay (PHLA) remains questionable because it has never been demonstrated that the bulk of endothelium-bound LPL was active. It has been recently shown that a small part of LPL is associated to circul ating lipoproteins in nonheparinized plasma, raising the possibility that t he lipolysis mediated by this circulating LPL might reflect the overall LPL -dependent TG hydrolysis in plasma. To address this question, we developed a new lipolysis assay in which the very low density lipoprotein (VLDL)-boun d LPL-dependent VLDL-TG hydrolysis (LVTH) was directly determined through t he measurement of nonesterified fatty acid (NEFA) release during in vitro i ncubations. LVTH measurements were performed in control subjects, in type 2 diabetics, and in either heterozygous or homozygous LPL-deficient patients . In the latter group, LVTH Values were extremely low. Those of heterozygou s patients and of diabetics were similarly decreased by about 40% with resp ect to control group. Plasma TG concentrations exhibited an inverse relatio nship with LVTH level. In a subgroup of subjects, LVTH and PHLA were positi vely correlated and the inverse correlation of LVTH with plasma or VLDL-TG concentration was stronger than that obtained with PHLA. To further study t he validity of this new assay, we measured LVTH in nine subjects who were s tudied for their catabolism of VLDL labeled with stable isotope. No relatio n was observed between the direct hepatic removal of VLDL and LVTH, whereas the latter was strikingly correlated with the rate of conversion of VLDL t o intermediary density lipoprotein. Collective consideration of these findings strongly suggests that LVTH is a physiologically relevant index which could advantageously replace the meas urements of PHLA in numerous physiopathological situations.