Hormonal activation of type-L hormone-sensitive lipase measured in defatted heart powders.

Adrenaline, 3-isobutyl-1-methylxanthine (MIX) and dibutyryl cyclic AMP (Bt2 cyclic AMP) stimulated type-L hormone-sensitive lipase (HSL) activity when measurements were made on defatted rat heart powders. These lipolytic agents stimulated the activity of this enzyme in a time- and dose-dependent manner. This activation was reversible, because removal of adrenaline from the perfusate was accompanied by the return of type-L HSL activity to control levels. We have reported [Palmer, Caruso & Oscai (1981) Biochem. J. 198, 159-166] that perfusion with low levels of adrenaline, MIX or Bt2 cyclic AMP reduced type-L HSL activity below control levels when measurements were made in aqueous homogenates. However, in the present study, when activities were measured in acetone/diethyl ether heart powders, all concentrations of these agents studied stimulated enzyme activity, and at no concentration was there enzyme inhibition. These data suggest that acetone/diethyl ether treatment may remove a factor that plays a role in type-L HSL regulation. Type-L HSL activity measured in acetone/diethyl ether powders of control and stimulated rat heart exhibited properties that include alkaline pH optimum, serum requirement, activation by heparin and inhibition by high salt and protamine sulphate. These characteristics, in addition to the stability of the enzyme to treatment with organic solvents, fulfil the requirements for the type-L HSL classification.