Hormonal regulation of protein dephosphorylation. Identification and hormonal regulation of protein phosphatase inhibitor-1 in rat adipose tissue.

An inhibitor (inhibitor-1) of phosphorylase a phosphatase has been identified in rat epididymal fat pads. This heat-stable, acid-soluble protein only exhibits phosphatase inhibitory activity when it itself is phosphorylated. Inhibitor-1 in rat adipose tissue migrates at 32,000 Da on sodium dodecyl sulfate-polyacrylamide gels, and at 64,000 Da on gel filtration. Exposure of fat pads to insulin (1 milliunit/ml) resulted in a 50% decrease in inhibitor-1 activity, compared to control (p less than 0.001). Isoproterenol (10(-6) M) caused a 25% increase in inhibitor-1 activity (p less than 0.05). Electrophoresis of heat-stable proteins prepared from hormone-treated 32P-labeled fat cells showed that insulin caused a dephosphorylation of the 32,000 Da phosphoprotein by 30% (p less than 0.01), whereas isoproterenol stimulated 32P incorporation in this protein by 35% compared to control (p less than 0.05). Thus, insulin appears to dephosphorylate and inactivate inhibitor-1, and might thereby result in an increase of protein phosphatase activity. Insulin regulation of inhibitor-1 is a mechanism which may underlie other of insulin's effects in adipose tissue, such as the activation of glycogen synthase.