MEASUREMENT AND PHYSIOLOGICAL SIGNIFICANCE OF LIPOPROTEIN AND HEPATICLIPASE ACTIVITIES IN PREHEPARIN PLASMA

A radiochemical method for selective measurement of postheparin lipase activities was adapted to analyze lipoprotein lipase and hepatic lipa se in preheparin plasma. The assay sensitivity was increased about fou rfold by doubling both the volume of plasma used and the volume of lip olytic products taken for liquid scintillation counting, and was furth er improved by increasing the incubation period by 50% to 90 min. Rabb it antiserum to human hepatic lipase was unsuitable for tile selective measurement of lipoprotein lipase because of apparent endogenous lipo lytic activity. Preheparin hepatic lipase, however, was sensitive to i nactivation by sodium dodecyl sulfate (SDS), the inhibition being grea test (>90%) for plasma incubated with an equal volume of 40 mmol/L SDS . Intra- and interassay CVs for the two enzymes were 12.5-14.6% and 17 .4-19.7%, respectively. In a; cross-sectional study of 84 healthy subj ects, pre- and postheparin hepatic lipase activities were higher in me n than women, were correlated with indices of obesity, and were signif icantly correlated with one another, which explained the association o f the former with plasma concentrations of high-density lipoprotein (H DL), HDL(2), and small, dense low-density lipoproteins. There was no s ignificant relationship between pre- and postheparin lipoprotein lipas e activities, but the former were correlated with plasma concentration s of free fatty acids (FFA) and very-low-density lipoprotein. Apparent ly, preheparin activities of hepatic lipase, but not of lipoprotein li pase, may be a useful measure of the physiological function of ''whole body'' enzyme activity in cross-sectional and metabolic studies, wher e heparinization is not possible. Preheparin lipoprotein lipase activi ties, however, may reflect displacement of the enzyme by FFA and subse quent binding to remnants of triglyceride-rich lipoproteins.