Purification and characterization of phosphorylase a from human platelets

Phosphorylation-induced conformational changes in the phosphorylase ab hybrid as revealed by resolution of pyridoxal 5'-phosphate with imidazole citrate and cysteine

The accessibility of pyridoxal 5'-phosphates of the phosphorylase ab hybrid to resolution by imidazole citrate and cysteine was studied and compared with that of the b and a forms. Promotion of resolution of phosphorylated forms by raising the temperature or in the presence of glycogen indicates that the resistance of phosphorylase a and ab to resolution at 0 degrees C is due rather to their tetrameric state than their phosphorylation-related active conformation. The pattern of resolution of the ab hybrid was similar to that of the a and differed from that of the b forms in that it occurred at 30 degrees C and 37 degrees C but not at 0 degrees C, moreover, it did not show first-order kinetics. On the other hand, inhibition of resolution by ligands binding to the nucleotide site of phosphorylase reflected an intermediate sensitivity of the ab form between that of the b and a forms. We conclude that partial phosphorylation of phosphorylase b elicits conformational change(s) in both subunits which influence the monomer-monomer interactions and resolution of pyridoxal 5'-phosphates. Resistance of ab hybrid to monomerizing agents as imidazole citrate, comparable to that of other forms, argues for its stability, ruling out its reshuffling into mixtures of phosphorylase b and a.

Glycogen phosphorylase in fish muscle: demonstration of three interconvertible forms

White skeletal muscle of crucian carp contains a single isoenzyme of glycogen phosphorylase, which was purified approximately 300-fold to a specific activity of approximately 13 mumol.min-1.mg protein-1 (assayed in the direction of glycogen breakdown at 25 degrees C). Tissue extracts of crucian muscle produced three distinct peaks of phosphorylase activity when separated on DEAE-Sephacel. Peaks 1 and 3 were identified, in terms of kinetic properties and by interconversion experiments, as phosphorylase b and a, respectively. Peak 2 was shown to be a phospho-dephospho hybrid. The three interconvertible forms of phosphorylase were purified and shown to be dimeric molecules at 20 degrees C. At 5 degrees C, a and the hybrid tended to form tetramers. The Mr of the subunit was estimated to be 96,400 from sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The hybrid is kinetically homogeneous, and its kinetic properties are intermediate between those of b and a forms. The b, hybrid, and a forms of phosphorylase can be isolated from rapidly frozen muscle of crucian but in different proportions, depending on whether fish were anesthetized or forced to muscular activity for 20 s. Muscle of anesthetized crucian had 36, 36, and 28% of phosphorylase b, hybrid, and a forms, respectively, whereas the corresponding values for exercised fish were 12, 37, and 51%. Results suggest that three interconvertible forms of phosphorylase exist simultaneously in crucian muscle and that hybrid phosphorylase is active in contracting muscle in vivo.

Kinetic characterization of rabbit skeletal muscle phosphorylase ab hybrid

Phosphorylase ab was prepared in vitro by partial phosphorylation of rabbit skeletal muscle phosphorylase b and was isolated by DEAE-Sephacel chromatography. Its phosphorylated and non-phosphorylated subunits could not be distinguished by different affinity to substrates, activators or inhibitors, indicating their coordinated function. In the absence of nucleotide activators, the Km values for Pi and glucose-1-P were 28 mM and 18 mM, respectively. Activity in the presence of 16 mM glucose-1-P was doubled by 10(-4) M AMP or 10(-3) M IMP, mainly by lowering the Km for glucose-1-P. Half-maximum activation was exerted by 2 microM AMP or 0.1 mM IMP. Activation by these nucleotides showed no cooperativity. Glucose exerted competitive inhibition with respect to glucose-1-P, while for the inhibition by glucose-6-P an allosteric mechanism is suggested; the appropriate Ki values were 4.5 mM and 1.5 mM, respectively. The Hill coefficient for glucose-1-P binding was about 1.0, even in the presence of glucose (up to 10 mM), but 10 mM glucose-6-P lowered it to 0.47, indicating a negative heterotropic cooperativity. Effective regulation of the activity of phosphorylase ab by physiological concentrations of Pi, AMP, IMP and glucose-6-P suggests its metabolic control under in vivo condition.

Phosphorylation of McArdle phosphorylase induces activity

In McArdle disease, myophosphorylase deficiency, enzyme activity is absent but the presence of an altered enzyme protein can frequently be demonstrated. We have found that phosphorylation of this protein in vitro can result in catalytic activity. We studied muscle of four patients; all lacked myophosphorylase activity, but myophosphorylase protein was demonstrated by immunodiffusion or gel electrophoresis. Incubation of muscle homogenate supernatants with cyclic AMP-dependent protein kinase and ATP resulted in phosphorylase activity. The activated enzyme comigrated with normal human myophosphorylase in gel electrophoresis. Incubation with [gamma-32P]ATP resulted in incorporatin of 32P into the band possessing phosphorylase activity. Activation of phosphorylase by cyclic AMP-dependent protein kinase was inhibited by antibodies to normal human myophosphorylase or by inhibitory protein to cyclic AMP-dependent protein kinase. Incubation of muscle homogenates with phosphorylase b kinase and ATP also resulted in phosphorylase activity. After the action of cyclic AMP-dependent protein kinase, the resulting activity was similar to that of phosphorylase b. However, incubation with phosphorylase kinase resulted in activity similar to that of phosphorylase a. For several reasons, it is not likely that McArdle disease is due to lack of normal phosphorylation, but restoration of activity to the mutant protein by phosphorylation may provide a clue to understanding the mechanism of this genetic defect.

Platelet phosphorylase kinase activity and its regulation by the calcium-dependent regulatory protein, calmodulin

Platelet phosphorylase kinase (ATP:phosphorylase phosphotransferase, EC 2.7.1.38) was found to be a Ca2+-sensitive enzyme. It was two Ka values for Ca2+ viz. 0.25 and 2.6 microM, respectively. The "calcium-dependent regulator" or calmodulin can enhance the activity of phosphorylase kinase, increasing its affinity for Ca2+. In the presence of calmodulin phosphorylase kinase has only one, high affinity binding site for Ca2+ (Ka = 0.27 microM). Platelet phosphorylase kinase can be phosphorylated by endogenous cyclic AMP-dependent protein kinase increasing its catalytic activity and this activation process is reversed by dephosphorylation. The changing level of intracellular Ca2+ and cyclic AMP may control the activity of phosphorylase kinase, regulating the mobilization of glycogen.