Postheparin plasma lipase in human subjects. Studies with chylomicrons, very-low density lipoproteins and a fat emulsion.

Abstract

Abstract. The properties of human postheparin lipolytic activity (PHLA) have been studied using an artificial triglyceride emulsion (Intralipid®), chylomicrons and endogenous lipoproteins as substrates. The assay system used requires only small amounts of plasma as the enzyme source and so reduces interference from the various activators and inhibitors present in plasma.The pH‐activity curves of PHLA were characteristically flat for all three substrates used, the optimum being between 8.4 and 9.0. The apparent Km (as triglyceride) was lower for chylomicrons than for very low density lipoproteins (VLDL) or Intralipid°. Wide variation was found, in the effects of protamine sulphate and 1 M NaCl on PHLA. This variation depended both on the donor of PHLA and on the type of substrate used.When the peak PHLA towards different substrates was compared in normal and hyperglyceridaemic subjects, an excellent correlation was found between the activities against chylomicrons and VLDL. The correlation between the activities towards endogenous substrates and Intralipid° was not so good. A significant negative correlation was observed between the fasting triglyceride level and lipolytic activity towards endogenous lipoproteins but not towards Intralipid°.No difference was found in the rates of disappearance of the lipolytic activities towards chylomicrons and VLDL after injection of heparin, when the results from 20 subjects were pooled. After a change from a high‐fat to a high‐carbohydrate diet PHLA decreased on average by 40%, irrespective of the substrate used.The results provide some further evidence for the presence of several triglyceride lipases in postheparin plasma. These upases may differ from each other both in substrate specificity and in their behaviour in the presence of various inhibitors.