Intracellular Distribution of Lipolytic Enzymes in the Rat Pancreas

Intracellular distribution of lipase (glycerolester hydrolase EC 3.1.1.3.), cholesterol esterase (sterol ester hydrolase EC 3.1.1.13.), phospholipase (EC 3.1.1.4.) and of carboxylic ester hydro‐lases (EC 3.1.1.1.) has been studied using isopycnic gradient centrifugation of rat pancreatic homogenates. The identity of the fractions obtained has been established using phase contrast and electron microscopy. Zymogen granules equilibrated at a mean density of 1.224 in aqueous sucrose gradients, mitochondria and cytomembranes at a mean density of 1.19 and 1.16–1.14, respectively. The zymogen fraction accounted for 25–45 % of the total activities of lipase, cholesterol esterase and phospholiphase. The specific activities (activity per mg protein) in the zymogen fraction were 5–7 times higher than those of the other cell fractions. A bile salt stimulated carboxylic ester hydrolase was found concentrated to the zymogen granules. The membrane and cell sap fractions accounted for almost all of the remaining lipolytic enzyme activities.