Selective Measurement of Two Lipase Activities in Postheparin Plasma from Normal Subjects and Patients with Hyperlipoproteinemia

An assay has been developed for specific measurement of two different lipase activities in postheparin plasma. Lipoprotein lipase, derived from extrahepatic sources, is measured as protamine-inactivated lipase activity; hepatic lipase activity is protamine-resistant under the conditions of this assay. In 100 normal subjects, both enzyme activities were noted to be related to age and sex. Protamine-resistant lipase, which comprised 46-95% of the total activity, was highest in men over 18. Protamine-inactivated lipase activity was greatest in younger males and was age-correlated in women, doubling between the second and sixth decades. In 12 patients with hyperchylomicronemia, including five previously shown to have familial type I hyperlipoproteinemia, protamine-inactivated lipase activity was markedly reduced, whereas protamine-resistant lipase was below normal in only 1. The results were not due to lack of plasma activator, presence of plasma inhibitor, or diet, and the deficiency was not overcome by increasing the provoking dose of heparin from 10 U to 75 U/kg. Mean values for both lipase activities were not reduced in 32 other patients with hyperchylomicronemia, nine with "floating beta" lipoproteins (type III hyperlipoproteinemia), and 23 with hyperprebetalipoproteinemia (type IV). Mean protamine-resistant lipase activity was below normal in a group of four women with hypothyroidism, in whom protamine-inactivated lipase was not reduced. Both of the lipase activities were capable of hydrolyzing lipid in very low-density lipoproteins, but the relative rate of hydrolysis of chylomicrons by protamine-resistant lipase was markedly limited. These results indicate the importance of distinguishing between lipases of hepatic and extra-hepatic origin in the measurement of postheparin lipolytic activity.