Characterization of insulin-sensitive phosphodiesterase in fat cells. II. Comparison of enzyme activities stimulated by insulin and by isoproterenol.

The stimulatory effects of insulin and certain lipolytic agents on CAMP phosphodiesterase were compared. The enzyme activity stimulated by insulin under optimum conditions did not increase any further by the addition of isoproterenol, adrenocorticotropin, dibutyryl CAMP, 3-isobutyl-l-methylxanthine, or theophylline. The basal, plus insulin, and plus isoproterenol activities were all associated with the endoplasmic reticulum. All three activities were maximum at about pH 7.5. The plus insulin and plus isoproterenol activities showed almost identical responses with heat, salt, and several sulfhydryl-blocking agents. With a mixture of Lubrol WX and Zwittergent 3-14, approximately 80% of the enzyme activity was solubilized. The solubilized enzyme retained the effects of insulin and isoproterenol. Upon gel filtration with Sepharose 4B, the solubilized enzyme was eluted mainly as a single symmetrical peak. The apparent Stokes radii of the solubilized plus insulin and plus isoproterenol enzymes were both approximately 94 A, while that of the basal enzyme was approximately 87 A. The difference in the elution volumes of the basal and plus hormone enzymes was statistically significant ( p < 0.01). It is suggested that although insulin and certain lipolytic agents (such as isoproterenol) have antagonistic physiological effects, they stimulate the same species of adipocyte phosphodiesterase and produce identical forms of the activated enzyme.