6 Hormone-Sensitive Lipase of Adipose Tissue

Abstract

Hormone-sensitive lipase (HSL) has been partially characterized in the adipose tissue of many species—including man, cow, pig, rat, and chicken. While patterns of hormonal control differ, the basic properties of the enzyme appear to be similar. This chapter mainly focuses on HSL enzyme in rat and chicken adipose tissue, where it has been extensively characterized. HSL has intrinsic activity against monoglyceride, and even more against diglyceride, although the rate-limiting reaction under all ordinary conditions is that against the intact triglyceride molecule. Because of the broad substrate specificity of the enzyme, assays may involve use of mono-, di-, or triglycerides or cholesterol esters. Although HSL is predominantly a cytosolic enzyme, a great deal of the enzyme tends to associate tightly with endogenous triglycerides and float to the top of the tube with them—the fraction of the total activity varies depending upon the ionic strength of the buffer in which the adipose tissue is homogenized. As much as two-thirds of the total activity may be associated with the fat cake when the tissue is homogenized in 0.15 M KCl. Purified preparations of HSL continue to show activity not only against triolein but also against diolein, monoolein, and cholesteryl oleate. Rat HSL in crude homogenates or in highly purified enzyme preparations is generally activated 50–150%, and almost never more than 200%. Yet the increase in the rate of lipolysis using isolated adipocytes can be as high as 20-fold.