Phosphorylase phosphatase

Phosphorylase phosphatase inhibitor Glutamate receptor agonist Unknown Na+, Ca++ channel activators... 

Biochemical site of action Site 1 on voltage-dependent sodium channel Site 5 on voltage-dependent sodium channel Catalytic subunit of phosphorylase phosphatases Kainate receptor in central nervous system Unknown Ciguatoxin site 5 on voltage-dependent sodium channel Maitotoxin calcium channels... 

Physiologic effect Inhibition of ion conductance Repetitive firing, shift of voltage dependence of activation Inhibition of phosphorylase phosphatases 1 and 2a Receptor-induced depolarization and excitation Unknown Ciguatoxin repetitive firing, shift of voltage dependence of activation Maitotoxin calcium ion influx... 

PHOSPHORYLASE KINASE PHOSPHORYLASE PHOSPHATASE PHOSPHORYLATION STATE RATIO 6-Phosphoryl-IMP,... 

FIGURE 6-31 Regulation of glycogen phosphorylase activity by covalent modification. In the more active form of the enzyme, phosphorylase a, specific Ser residues, one on each subunit, are phosphorylated. Phosphorylase a is converted to the less active phosphorylase b by enzymatic loss of these phosphoryl groups, promoted by phosphorylase phosphatase. Phosphorylase b can be reconverted (reactivated) to phosphorylase a by the action of phosphorylase kinase. 

The phosphoryl groups can be hydrolytically removed by a separate enzyme called phosphorylase phosphatase ... 

The 5-nitrofuryl group225 is promulgated as an efficacious moiety to ensure chemotherapeutic value, and the trans derivative (135) was found to be active against Staphylococcus aureus,171 It is also a broad-spectrum antimicrobial agent226 and an antischistosomal drug.227 The latter activity is due to reduction of the phosphorylase phosphatase in the worms themselves, damaging reproduction in the female.227, 228... 

Rabbit muscle phosphorylase can exist in two forms an essentially inactive dimer, phosphorylase b, and an active tetramer, phosphorylase a. When AMP is noncovalently bound to phosphorylase b. or when phosphorylase b is phosphorylated at a serine residue by phosphorylase kinase, the enzyme is converted to the active, predominantly tetrameric, form (Chap. 11). The reaction can be reversed by the removal of AMP or the dephosphorylation of serines by phosphorylase phosphatase. 

The conversion of phosphorylase b to a involves the addition of the terminal phosphate of ATP. The phosphate is then removed as the enzyme is converted back to phosphorylase b under the catalytic influence of phosphorylase phosphatase (EC 3.1.3.17 phosphorylase phosphohydrolase). Phosphorylase phosphatase from rabbit skeletal muscle has been purified 700-fold [86]. 

The first assay procedure to be reported [23] and which is still being used with minor modifications [6] employs dog liver phosphorylase. The preparation of the enzyme entails the isolation of active phosphorylase and subsequent inactivation with phosphorylase phosphatase. Inactive dog liver phosphorylase is stable for many years when frozen at - 20°C. 

Phosphorylase phosphatase (EC 3.1.3.17 phosphorylase phosphohydrol-ase) catalyses the conversion of phosphorylase a to phosphorylase b with the release of inorganic phosphate The reaction can be followed by the disappearance of phosphorylase a activity or by the release of radioactive phosphate from P-labelled phosphorylase a [167]. 

The above may be of pathophysiological relevance as a decreased basal and insulin-stimulated glycogen phosphatase activity and phosphorylase phosphatase activity (Type-1 protein phosphatase, PP-1) in the skeletal muscle of insulin-resistant individuals has been found (Freymond era/., 1988 Kida era/., 1990, 1992 Damsbo et al., 1991). Other studies suggest increased phosphotyrosine phosphatase activity in patients with NIDDM who show decreased autoactivation of the insulin receptor kinase (McGuire et al., 1991). 

Regulation of glycogen phosphorylase in muscle is accomplished by many of the same enzymes that control glycogen synthesis. Phosphorylase kinase converts the dimeric phosphorylase from the inactive to the active form by Mg + and ATP-dependent phosphorylation of two identical serine residues. The principal enzyme that removes this phosphate may be protein phosphatase-1 (phosphorylase phosphatase). 

The mode of action of this drug is not fully understood. It appears that niridazole inhibits phosphorylase phosphatase with subsequent potentiation of phospho-rylase. This may result in depletion of glycogen levels in the schistosomes. Thus, the major action of niridazole seems to be on the glycogen metabolism of the helminths. The drug also causes structural damage to the reproductive system of female schistosomes [95,96]. 

Waelkens E, Goris J, Merlevede W (1987) Purification and properties of polycation-stimulated phosphorylase phosphatases from rabbit skeletal muscle. J Biol Chem 262 1049-1059... 

Cheng Q, Erickson AK, Wang ZXetal (1996) Stimulation of phosphorylase phosphatase activity of protein phosphatase 2A1 by protamine is ionic strength dependent and involves interaction of protamine with both substrate and enzyme. Biochemistry 35 15593-15600... 

Agostinis P, Goris J, Waelkens E et al (1987) Dephosphorylation of phosphoproteins and synthetic phosphopeptides. Study of the specificity of the polycation-stimulated and MgATP-dependent phosphorylase phosphatases. J Biol Chem 262 1060-1064... 

Glucagon-initiated stimulation of glycogen phosphorylase. PKA denotes protein kinase A, and the subscripts / and a refer to active and inactive, respectively R denotes the regulatory subunit and PPl denotes glycogen phosphorylase phosphatase 1. 

The regulation of protein phosphatases is intimately related to that of protein kinases, both being part of an overall cycle that acts as a switching mechanism to turn-on or turn-off certain enzyme activities. Within the past decade, at least 20 publications have described the involvement of Mn(II) with these systems. Most notably, these include the following protein-specific enzymes phosphorylase phosphatase [87,104], Ga/calmod-ulin-dependent protein phosphatase [94,99,105-107], and calcineurin phosphatase [91,93,95], each being involved in intracellular signalling and metabolic regulation. 

Glycogen phosphorylase (14.48) mw = 370,000, exists in active (A) and relatively inactive (B) forms which differ in quaternary structure and phosphate content. The active phosphorylase (A) contains two phosphate groups bound to two serine units, out of a total of 30 serine units in each protein molecule. This enzyme is hydrolysed by phosphorylase phosphatase to produce the inactive (B) form which is devoid of phosphate groups. Re-phosphorylation to produce the active (A) form can be achieved with ATP and phosphorylase kinase. 

Carboxylase activation by phosphorylase phosphatase is inhibited by the specific inhibitory proteins for phosphorylase phosphatase, indicating such activation is the result of phosphatase action and not of the proteol3itic activity 41, 60). 

Reports that the activation of acetyl-CoA carboxylase from rat epididymal fat tissues 60) and rabbit mammary glands 41) by liver phosphorylase phosphatase is accompanied by dephosphorylation, and that both events are blocked by rat liver phosphatase inhibitor-1, provide compelling evidence for the occurrence of the covalent modification mechanism in the control of the activity of the carboxylase. However, isolation and studies of the properties of specific enzymes for the interconversion are essential for our understanding of the mechanism... 

Glycogen phosphorylase phosphatase is involved in carbohydrate metabolism of S. mansoni and inhibition of this enzyme by niridazple ultimately affects the reproductive system of the female schistoscmie. 

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