Paavo Kinnunen

A simple purification procedure for rat hepatic lipase

Lipoprotein lipase (LPL, EC 3.1 .1.3.) of the extrahepatic tissues is responsible for the hydrolysis of triacylglycerols transported in plasma chylomicrons and very low density lipoproteins [ 1,2]. This exoenzyme can be released into the circulation by intravenous injection of heparin or similar poiyanions [3]. In addition to the lipoprotein lipase, post-heparin plasma contains a triacylglycerol lipase of hepatic origin [4]. The hepatic lipase also hydrolyzes monoacyi~ycerols and phospholipids [5,6]. The role of the hepatic lipase in the lipoprotein metabolism is not known. It has been suggested that the enzyme could participate in the hydrolysis of the lipids in ‘retnnant’ lipoproteins [7]. The activity of the hepatic lipase is not lowered in hypertri~yceridaemia, and it does not correlate to the plasma level of triacylglycerol [ 11. Accordingly, the hepatic lipase cannot be the rate-lil~iting factor in plasma triacyl~ycerol removal. It has also been proposed that the hepatic lipase could mediate the breakdown of lipoprotein monoacylglycerols [8,9]. As the role of the hepatic lipase in lipoprotein metabolism is far from resolved, we felt it necessary to purify the enzyme in sufficient quantities to enable unambi~ous in vitro studies. We now describe a simple purification procedure for the rat hepatic lipase using heparin-containing liver perfusates as a starting material.

Studies on the effect of hepatectomy on pig post-heparin plasma lipases

Abstract The effect of different amounts of heparin injected intravenously in swine on lipoprotein lipase and hepatic lipase activities in post-heparin plasma was studied using an immunochemical method. After the injection of 50 I.U. of heparin/kg body weight the apparent half-life of lipoprotein lipase and hepatic lipase activity measurable in post-heparin plasma was 15 min. This was prolonged to more than 60 min after the injection of 1000 I.U./kg body weight. It is concluded that the higher the heparin dose injected the longer can lipolytic activities be measured in plasma. A possible explanation for these findings is that the amount of circulating heparin governs the distribution of lipoprotein lipase and hepatic lipase between an endothelial-bound form and a circulating form and thus determines the apparent ‘half-life’ of lipase activity measurable in plasma. The apparent half-life of radioactively labelled heparin in normal swine was not different from that observed in hepatectomized animals. After hepatectomy no immunoreactive hepatic lipase activity could be demonstrated in post-heparin plasma confirming our previous findings that the liver is the only source of hepatic lipase. To study the role of the liver in the clearance of plasma lipoprotein lipase activity after the administration of heparin normal and hepatectomized pigs were given 200 I.U./kg body weight followed by a heparin infusion of 100 I.U./ h per kg body weight. In the control pigs the heparin injection caused a rapid release of lipoprotein lipase and hepatic lipase activities. These activities were maintained in the circulation during the 3-h infusion at a level of about 60% of the levels measurable 30 min after the injection. In hepatectomized pigs the lipoprotein lipase activity rose during the infusion to about six times the activity recorded 30 min after heparin administration. From these experiments we conclude that after heparin injection the liver is involved in the clearance of post-heparin plasma lipolytic activity.