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Glycogen synthase is activated by protein phosphatase type-1 (PP-1). The spontaneous PP-1 activity accounts for only a small fraction of total PP-1 activity, which can be exposed by trypsin digestion of inhibitor proteins in the presence of Mn2+. We determined total PP-1 activity in muscle biopsies from insulin-sensitive and -resistant nondiabetic Pima Indians. Inhibitor-2 sensitive PP-1 represented 90% of total phosphatase activity. Spontaneous and total PP-1 activities were reduced in insulin resistant subjects (P less than 0.05-0.01), suggesting that the reduced PP-1 activity is not the result of inhibition by trypsin-labile phosphatase regulatory subunits. This difference was further investigated by Western blots using two different antibodies. An antibody raised against the rabbit muscle PP-1 catalytic subunit was used to analyze muscle extracts concentrated by DEAE-Sepharose adsorption. An antibody raised against a peptide derived from the COOH-terminal end of the PP-1 catalytic subunit was used to analyze crude muscle extracts. Both antibodies recognized a PP-1 catalytic subunit of approximately 33 kD, which unexpectedly was more abundant in insulin-resistant subjects (P less than 0.05-0.01). The increase in the tissue PP-1 protein content may be a response to compensate for the impairment in the enzyme activity.

Acetyl CoA carboxylase was purified from liver of fasted-refed rats to near homogeneity, based on electrophoretic analysis and biotin content. These preparations contained an endogenous protein kinase that catalyzed the transfer of radioactive phosphate from [gamma-32P]ATP to acetyl CoA carboxylase, accompanied by a decrease in acetyl CoA carboxylase activity. Phosphate incorporated into acetyl CoA carboxylase was removed when the preparation was incubated with partially purified phosphorylase phosphatase catalytic subunit with regain of enzymatic activity. This endogenous protein kinase was shown not to be affected by either cyclic-AMP-dependent protein kinase inhibitor, EGTA, or trifluoperazine. The addition of either cyclic-AMP or purified cyclic-AMP-dependent protein kinase catalytic subunit to the purified acetyl CoA carboxylase preparation increased protein phosphorylation but had no further effect on acetyl CoA carboxylase activity. Purified acetyl CoA carboxylase was shown to act as an ATPase during the phosphorylation reaction.

A detailed investigation was conducted to determine the precise subcellular localization of the rate-limiting enzymes of hepatic glycogen metabolism (glycogen synthase and phosphorylase) and their regulatory enzymes (synthase phosphatase and phosphorylase phosphatase). Rat liver was homogenized and fractionated to produce soluble, rough and smooth microsomal fractions. Enzyme assays of the fractions were performed, and the results showed that glycogen synthase and phosphorylase were located in the soluble fraction of the livers. Synthase phosphatase and phosphorylase phosphatase activities were also present in soluble fractions, but were clearly identified in both rough and smooth microsomal fractions. It is suggested that the location of smooth endoplasmic reticulum (SER) within the cytosome forms a microenvironment within hepatocytes that establishes conditions necessary for glycogen synthesis (and degradation). Thus the location of SER in the cell determines regions of the hepatocyte that are rich in glycogen particles. Furthermore, the demonstration of the association of synthase phosphatase and phosphorylase phosphatase with membranes of SER may account for the close morphological association of SER with glycogen particles (i.e., disposition of SER membranes brings the membrane-bound regulatory enzymes in close contact with their substrates).

The administration of glucose to anesthetized fed rats causes within 1 to 2 minutes the inactivation of phosphorylase; glycogen synthetase is activated only when and if the level of phosphorylase α has been taken down below a threshold value equal to approximately 10% of the total phosphorylase. The same conclusion has been reached in a study with anesthetized fed mice. These observations are adequately explained by the previously described stimulation of the phosphorylase phosphatase reaction by glucose and inhibition of synthetase phosphatase by phosphorylase α. In some experiments, insulin caused a partial inactivation of liver phosphorylase in the liver of normal rats, but more often was without effect. When effective in the diabetic animal, insulin produced the inactivation of glycogen phosphorylase and the activation of glycogen synthetase. Glucocorticoids displayed two effects: a greater activity of phosphorylase phosphatase and a decreased inhibition of synthetase phosphatase by phosphorylase α.

The gamma1-isoform of protein phosphatase-1 expressed in Escherichia coli (PP1gamma) and the native PP1 catalytic subunit (PP1C) isolated from skeletal muscle dephosphorylated Ser-14 of glycogen phosphorylase at comparable rates. In contrast, PP1gamma dephosphorylated several tyrosine-phosphorylated proteins at similar rates to authentic protein tyrosine phosphatases (PTPases), but native PP1C was almost inactive towards these substrates. The phosphorylase phosphatase (PhP) and PTPase activities of PP1gamma were inhibited by vanadate with IC50 values (30-100 microM) comparable to authentic PTPases, whereas the PhP activity of native PP1C was insensitive to vanadate. PP1gamma lost its PTPase activity, and its PhP activity became insensitive to vanadate, after interaction with inhibitor-2, followed by the reversible phosphorylation of inhibitor-2 at Thr-72. These findings support and extend the hypothesis that inhibitor-2 functions like a chaperone to fold PP1 into its native conformation, and suggest that the correct folding of PP1 may be critical to prevent the uncontrolled dephosphorylation of cellular phosphotyrosine residues.

Proteins of 60–70 kDa copurify with some preparations of type-1 or type-2 phosphatases. In our system chromatography on polylysine-Affi-Gel 10 separates a 68 kDa protein from rabbit muscle glycogen particle phosphorylase phosphatase. The separation affects neither the activity nor the size of the phosphatase. The 68 kDa protein, although pure by SDS gel electrophoresis criteria, still displays phosphatase activity of approx. 6–8 Umg. However, rechromatography either on Bio-Gel A-0.5 m or on Blue Sepharose CL-6B followed by gel filtration shows that the activity is due to a contamination with phosphatases of type 1 and type 2, displaying a molecular mass of 35 kDa, which can be totally removed from the 68 kDa protein. The amino acid composition of the 68 kDa protein is identical to that of rabbit serum albumin, within the limits of variation of the method. Furthermore, the sequence of the 38 N-terminal amino acids is the same in the isolated 68 kDa protein and in rabbit serum albumin.

Different enzymes, related to the glycogen metabolism have been reported to be associated in the rabbit skeletal muscle within a ‘glycogen particle’ [l-5]. In this purified particle one can detect the presence of glycogen phosphorylase [ 1,351, phosphorylase kinase [ 1,5] , phosphorylase phosphatase [l] , amylase [l] , myokinase [l] , adenylic deaminase [ 11, the enzymes of the glycolytic pathway [l] , g$xgen synthetase [3,5] and amylo I-6-glucosidase

The dephosphorylation of the modulator subunit is an essential step in the kinase FA-mediated activation of the ATP,Mg-dependent protein phosphatase. Mg2+ is implicated in this autocatalytic dephosphorylation which is not effected by the addition of phosphoinhibitor-1. Dephosphorylation of free modulator by the catalytic subunit is also largely Mg2+-dependent but can be abolished by phosphoinhibitor-1 in concentrations comparable to the amount of modulator used as substrate (micromolar). The phosphorylase phosphatase activity of the catalytic subunit is inhibited by nanomolar concentrations of phosphoinhibitor-1 and is completely independent of divalent cations.

A phosphate-acceptor protein was isolated from the skeletal muscle of the Pacific dogfish ( Squalus acanthias ) displaying properties extremely similar to those of the parvalbumins, i.e., the low-molecular-weight, soluble, Ca-binding muscle proteins found in fish and amphibians. It has the same characteristic UV spectrum, strong affinity for calcium, and immunological crossreactivity with antibodies against homogeneous dogfish parvalbumin. Although it was isolated in three states of aggregation with molecular weights of about 350,000, 75,000, and 25,000, all species dissociate in Na dodecyl sulfate into subunits of 11,000 and 13,000 molecular weight. Furthermore, whereas no phosphorylation of parvalbumins could be demonstrated under any experimental conditions, the aggregated forms could be readily phosphorylated by a cyclic AMP-independent dogfish protein kinase, but not by phosphorylase kinase. One acid-stable and base-labile phosphate group was introduced per subunit which could be rapidly released by a dogfish protein phosphatase, but only very slowly if at all by phosphorylase phosphatase. It is speculated that this “phosphate-acceptor protein” might represent a physiologically active form of the parvalbumins.

Extracts of Brassica napus (oilseed rape) seeds contain type 1 and type 2A protein phosphatases whose properties are indistinguishable from the corresponding enzymes in mammalian tissues. The type 1 activity dephosphorylated the beta-subunit of phosphorylase kinase selectively and was inhibited by the same concentrations of okadaic acid [IC50 (concentration causing 50% inhibition) approximately 10 nM], mammalian inhibitor 1 (IC50 = 0.6 nM) and mammalian inhibitor 2 (IC50 = 2.0 nM) as the rabbit muscle type 1 phosphatase. The plant type 2A activity dephosphorylated the alpha-subunit of phosphorylase kinase preferentially, was exquisitely sensitive to okadaic acid (IC50 approximately 0.1 nM), and was unaffected by inhibitors 1 and 2. As in mammalian tissues, a substantial proportion of plant type 1 phosphatase activity (40%) was particulate, whereas plant type 2A phosphatase was cytosolic. The specific activities of the plant type 1 and type 2A phosphatases were as high as in mammalian tissue extracts, but no type 2B or type 2C phosphatase activity was detected. The results demonstrate that the improved procedure for identifying and quantifying protein phosphatases in animal cells is applicable to higher plants, and suggests that okadaic acid may provide a new method for identifying plant enzymes that are regulated by reversible phosphorylation.

We investigated the inhibitory effect of Ca2+ in the micromolar range on the activation of glycogen synthase in crude gel-filtered liver extracts [van de Werve (1981) Biochem. Biophys. Res. Commun. 102, 1323-1329]. The magnitude of the inhibition was highly dependent on the glycogen concentration in the final liver extract. Ca2+ inhibited the activation of purified hepatic synthase b by the G-component of synthase phosphatase, as present in the isolated glycogen-protein complex. The cytosolic S-component was not inhibited. Maximal inhibition of the crude G-component occurred at 0.3 microM-Ca2+. The inhibition was not influenced by the addition of either calmodulin or calmodulin antagonists, or by various proteinase inhibitors. The use of purified G-component revealed that the inhibition by 0.3 microM-Ca2+ increased from 45% to 85% when the concentration of glycogen was raised from 1.5 to 20 mg/ml. Muscle glycogen synthase, extensively phosphorylated in vitro, was also used as substrate for purified G-component. Activation and dephosphorylation were similarly inhibited by 0.3 microM-Ca2+, but the magnitude of the inhibition was much greater with the hepatic substrate. No effect of 0.3 microM-Ca2+ was found on the activity of phosphorylase phosphatase in various liver preparations. We conclude that the inhibition of synthase activation by Ca2+ is one of the mechanisms by which cyclic AMP-independent glycogenolytic hormones promote the inactivation of glycogen synthase in the liver, especially in the fed state.

The activity of glycogen phosphorylase (1,4-alpha-D-glucan:orthophosphate alpha-D-glucosyltransferase, EC 2.4.1.1) is controlled by a cyclic phosphorylation-dephosphorylation process through the action of the interconverting enzymes, phosphorylase b kinase (ATP:phosphorylase-b phosphotransferase, EC 2.7.1.38) and phosphorylase a phosphatase (phosphorylase a phosphohydrolase, EC 3.1.3.17). In muscle tissue, the combined concentration of the activated (phospho-) form, phosphorylase a, and the nonactivated (dephospho-) form, phosphorylase b, is substantially greater than the Km of either of the interconverting enzymes for its phosphorylase substrate. It has been predicted that, under such a set of conditions, a sensitivity amplification will occur for phosphorylase regulation due to the zero-order ultrasensitivity effect [LaPorte, D. C. & Koshland, D. E., Jr. (1983) Nature (London) 305, 286-290]. The sensitivity amplification will enhance the responsiveness of the phosphorylase interconversion cycle to changes in the ratio of activities of the kinase to phosphatase. We have studied the cyclic interconversion process using purified muscle enzymes in steady-state reactions and found that there is an enhancement in the control sensitivity of the process due to the zero-order ultrasensitivity effect. The potential for the in vivo enhancement of sensitivity in glycogen degradation by this effect is discussed.

The kinetic properties and the control mechanism of fructose-6-phosphate 2-kinase (ATP: D-fructose-6-phosphate 2-phosphotransferase) were investigated. The molecular weight of the enzyme is approximately 100,000 as determined by gel filtration. The plot of initial velocity versus ATP concentration is hyperbolic with a Km of 1.2 mM. However, the plot of enzyme activity as a function of fructose-6-phosphate is sigmoidal. The apparent K0.5 for fructose-6-phosphate is 20 microM. Fructose-6-phosphate 2-kinase is inactivated by the catalytic subunit of cyclic AMP-dependent protein kinase, and the inactivation is closely correlated with phosphorylation. The enzyme is also inactivated by phosphorylase kinase in the presence of Ca2+ and calmodulin. The phosphorylated fructose-6-phosphate 2-kinase, which is inactive, is activated by phosphorylase phosphatase and alkaline phosphatase. The possible physiological significance of these observations in the coordinated control of glycogen metabolism and glycolysis is discussed.