Insulin-induced increases in the activity of the spontaneously active and ATP . Mg-dependent forms of phosphatase-1 in alloxan-diabetic rat liver

Abstract

Liver supernatant from normal and alloxan-diabetic rats was fractionated by DEAE-cellulose chromatography and the separated phosphoprotein phosphatase fractions were assayed with [32P]histone fzb, [32P]phosphorylase a and [32P]phosphorylase kinase as substrates. In diabetic rat liver, one of the phosphatase fractions found in the normal liver was significantly reduced. This fraction was identified as a mixture of the spontaneously active form and the ATP . Mg-dependent form of phosphoprotein phosphatase-I (F,) based on sensitivity to inhibitor-2, substrate specificity, and the fact that it could be activated 42 -70% by glycogen synthase kinase-3 in the presence of ATP . Mg. Further analysis of this fraction showed that liver cytosol from diabetic rats contained 62-79% lower spontaneously active phosphatase-I activity and 40 -51 % lower combined spontaneously active and ATP . Mg-dependent protein phosphatase-1 (F,) activity. Insulin administration increased the spontaneously active and the ATP . Mg-dependent protein phosphatase-I activities approximately 45% and 36%, respectively, in alloxan-diabetic rats. These data imply that the lower levels of spontaneously active phosphatase-1 activity in diabetic rat liver cannot be explained by presuming phosphatase-1 to have been present as Fc, the inactive form. Moreover, insulin restored the total activity of the spontaneously active and activatable forms of phosphatase-I to those present in normal liver implying that both forms of phosphatase-I activity are under hormonal control.

The Structure and Regulation of Protein Phosphatases

“…There was no change in PP-} activity in skeletal muscle and no change in hepatic or muscle PP-2A. Administration of insulin to the diabetic animals for only five min was reported to increase active PP-} by 45% and activatable PP-ll by 36% (313). A larger (62-79%) decrease in the phosphory lase phosphatase activity of rat hepatic PP-I in alloxan diabetic animals, with no change in PP-2A, was reported by others (3 13).…”

Mechanistic aspects of the low-molecular-weight phosphatase activity of calcineurin: a model for phosphotyrosyl-protein phosphatases

“…The phosphatases included in this scheme can account for the majority of phosphatase activity involved in the regulation of metabolic pathways (77)(78)(79)(80)(81)85). It is well established that phosphatase 1, or phosphorylase phosphatase, is subject to hormonal regulation (82,(105)(106)(107)(108). These hormonal effects on phosphatase 1 are modulated through the function of one of the two phosphatase inhibitor proteins specific for this phosphatase.…”