Effect of colchicine, cycloheximide and chloroquine on the hepatic triacylglycerol hydrolase in the intact rat and perfused liver

Abstract

Hepatic triacylglycerol hydrolase activity was studied in the intact rat and in isolated perfused liver. Using cycloheximide to inhibit protein synthesis the t 1 2 of total enzyme activity in the liver was 4.6 h as determined in the intact rat and after liver perfusion. With the help of non-recirculation perfusion the heparin-releasable fraction of liver triacylglycerol hydrolase was shown to comprise about 30% of total activity and maximal rates of release were obtained within 2 min of perfusion. Following pretreatment with colchicine this fraction was reduced to only 13%, while total enzyme activity remained the same as in controls. Contrary to these findings, the fall in total enzyme activity 3 h after cycloheximide was accounted for mainly by a fall in residual activity, while the heparin-releasable activity fell only between 3 and 6 h after inhibition of protein synthesis. These findings provide direct evidence that this triacylglycerol hydrolase is synthesized in the liver, is delivered to the cell surface through a vesicular transport system and that this transport continues to operate 3 h after inhibition of protein synthesis. With the use of chloroquine, support was obtained that lysosomal enzymes may participate in the degradation of the enzyme. A hypothesis was proposed that the enzyme resides near a lipoprotein remnant receptor and that it may be interiorized during remnant endocytosis.