Characterization of tissue lipolytic and esterolytic activities cleaving full and partial glycerides

Abstract

Rat liver, kidney, heart, diaphragm, lung and spleen were found to contain lipolytic activities with increasing rates of cleavage from tri- to monoglycerides of long-chain fatty acids. These activities were assumed to be due to distinct enzymes, as inferred from varying ratios of activity in different tissues, different pH optima and inhibition characteristics. Similarities between tri- and monoglyceride lipase were found in their prevalence in the microsomal fraction of tissue homogenates and in the inhibitory effects on their activity of homogenization in the presence of ATP, cations, especially divalent, and of preincubation with uncouplers of oxidative phosphorylation. Thus, tri- and monoglyceride lipases seem to reside in the same intracellular site and are activated in a similar fashion, but upon homogenization exhibit distinct properties. Cleavage of tri- and monoglycerides of short-chain fatty acids, exemplified by tri- and monobutyrin, did not show the differences encountered between tri- and monoolein. Cleavage of the full and partial butyrate esters was, therefore, attributed to an esterase distinct from the lipases, as seen in the varying ratio of activity between esterolytic and lipolytic activities among the tissues studied, in pH optimum, susceptibility to inhibitors and in the kinetics of cleavage at rising substrate concentration. In analogy to lipases, the esterolytic activity was located in the microsomal fraction and was also somewhat affected by ATP and ions during tissue homogenization, suggesting a common location and similar mode of activation. The implication of lipase content in non-adipose tissues of the rat and the role of ATP and cations in their activation are briefly discussed in conjunction with the lipase activity in adipose tissue.