Neutral and alkaline lipolytic activities in human adipose tissue

Abstract

Previous work from this laboratory suggested that human adipose tissue contains at least 2 lipolytic activities. Assay systems have been developed for the optimal demonstration of these activities in cell-free preparations of human adipose tissue, and additional differences have been demonstrated. Neutral lipolytic activity was assayed at pH 7.0, 37° with olive oil as substrate, and alkaline lipolytic activity at pH 8.0, 47° with tributyrin as substrate. Studies which related triglyceride concentration to both lipolysis and absorbance of the assay system suggested that neutral lipolytic activity represents hydrolysis of an emulsified ester and alkaline lipolytic activity hydrolysis of a soluble ester. In general, neutral lipolytic activity was augmented considerably by albumin, 50 mg/ml, but alkaline activity was not. In addition to the differences in assay conditions the 2 activities differed in their thermal stability, pH dependence, and inhibition characteristics. These activities differ in their hydrophilic tendencies as seen by the fact that they were partially separated by centrifugation of an adipose tissue homogenate, or by extraction of an adipose tissue preparation with diethyl ether. In addition each activity differs from lipoprotein lipase activity.