Muscle phosphorylase activation

A defect in glycogen metabolism confined to muscle is found in McArdle disease (type V). Muscle phosphorylase activity is absent, and the patient s capacity to perform strenuous exercise is limited because of painful muscle cramps. The patient is otherwise normal and well developed. Thus, effective utilization of muscle glycogen is not essential for life. The results of phosphorus-31 nuclear magnetic resonance studies of these patients have been very informative. The... 

In the Type VI diseases, there appears to be abnormally low or complete absence of phosphorylase activity. In three cases (Type Via) examined by Hers, the activity of the liver phosphorylase was about 5 units, compared with values of 12-33 units for other cases of glycogen-storage disease, although the muscle phosphorylase activity was normal. 

Glycogen phosphorylase has been purified from adult A. suum muscle (3). The dephosphorylated enzyme, which requires AMP for activity, can be phosphorylated and activated by rabbit muscle phosphorylase kinase. Kinetic data and the observation that phosphorylase is not at equilibrium with its substrates and products, indicate that phosphorylase catalyzes the rate-limiting step of glycogenolysis in A. suum muscle. Phosphorylase activity correlates well with the rate of glycogenolysis observed in vivo... 

This is due to the absence of muscle phosphorylase activity. Phos-phorylase is one of the glycolytic enzymes. 

The gene for myophosphoiylase has been assigned to chromosome 1 lql3. The enzyme is a dimer of two identical 97 IcDa subunits and is the sole isoform present in skeletal muscle. Heart and brain also contain this isoform in addition to a distinct brain isoenzyme and a hybrid muscle/brain isoform. Smooth muscle also contains a phosphorylase isoform distinct from the muscle isoenzyme. If regenerating muscle fibers are present they also contain phosphorylase activity due to the presence, in fetal and developing muscle, of an isoform said to be identical with brain phosphorylase. 

Muscle phosphorylase is distinct from that of Hver. It is a dimer, each monomer containing 1 mol of pyridoxal phosphate (vitamin Bg). It is present in two forms phos-phoiylase a, which is phosphorylated and active in either the presence or absence of 5 -AMP (its allosteric modifier) and phosphorylase h, which is dephosphorylated and active only in the presence of 5 -AMP. This occurs during exercise when the level of 5 -AMP rises, providing, by this mechanism, fuel for the muscle. Phosphorylase a is the normal physiologically active form of the enzyme. 

Phosphorylase in muscle is activated in response to epinephrine (Figure 18-6) acting via cAMP. Increasing the concentration of cAMP activates cAMP-dependent... 

Glycogenolysis increases in muscle several hundred-fold immediately after the onset of contraction. This involves the rapid activation of phosphorylase by activation of phosphorylase kinase by Ca +, the same signal as that which initiates contraction in response to nerve stimulation. Muscle phosphorylase kinase has four... 

Phosphorylase was studied in depth. The enzyme from muscle was different from that catalyzing the same reaction in liver. Muscle phosphorylase but not that from liver, was activated by AMP, an early example of enzyme regulation by a ligand which was not a substrate. [Allosteric regulation was not postulated until the work of Jacob and... 

Fig. 8. The activity of muscle phosphorylase b (as a logarithm percentage of the activity of the fully AMP-activated enzyme in water) as a funaion of concentration (% v/v) of added organic cosolvent in the absence of AMP. It can be seen that long

In the 1940s Carl and Gertrude Cori isolated and purified an active form (phosphorylase a) and an inactive form (phosphorylase b) of an enzyme from muscle. Phosphorylase b is activated by AMP (see page 64). In 1955, Fischer Krebs found an enzyme that catalysed the conversion of phosphorylase b to phosphorylase a, together with hydrolysis of ATP to ADP. Thus it appeared to bring about phosphorylation of the enzyme. The enzyme was termed phosphorylase b kinase, was partially purified and the interconversion was established as... 

Figure 3.12 The regulation of phosphorylase activity by reversible phosphoiylation. A reversible phosphorylation process is also known as an interconversion cycle the latter term is preferred in this text, since the individual reactions must be irreversible, which can be confusing if the term reversible is used to describe the overall process. In resting muscle, almost all phosphorylase is in the b form.

High resolution X-ray structures of the a- and b- form of rabbit muscle phosphorylase permit a view into some of the structural differences of the various allosteric forms of the enzyme. Furthermore, the data give an impression of the mechanism of binding of effectors and the influence that phosphorylation has on substrate binding and enzyme activity (Barford et al., 1991). The following discussion will be restricted to the observed consequences of phosphorylation. 

Krebs, E.G., Graves,D.J. and Fischer, E.H. Factors affecting the activity of muscle phosphorylase kinase (1959) J. Biol. Chem. 2867-2873... 

Fig. 7.18. Regulation of glycogen metabolism in muscle. Phosphorylase kinase stands at the center of regulation of glycogen metabolism. Phosphorylase kinase may exist in an active, phosphorylated form and an inactive, unphosphorylated form. Phosphorylation of phosphorylase kinase is triggered by hormonal signals (e.g. adrenahne) and takes place via an activation of protein kinase A in the cAMP pathway. In the absence of hormonal stimulation, phosphorylase kinase can also be activated by an increase in cytosolic Ca. The active phosphorylase kinase stimulates glycogen degradation and inhibits glycogen synthesis, in that, on the one side, it activates glycogen phosphorylase by phosphorylation, and on the other side, it inactivates glycogen synthase by phosphorylation.

Fig. 7.19. Subunit structure and regulation of phosphorylase kinase of muscle. Phosphorylase kinase is - according to the excitation state of the muscle - regulated by two pathways. On nervous stimulation of the muscle, voltage-controlled Ca channels are opened, the cytosolic Ca concentration increases and Ca binds to calmoduhn, activating phosphorylase kinase. In resting muscle, activation of phosphorylase kinase is triggered by a hormonal signal. A hormonal signal initiates phosphorylation of the a and P subunits of phosphorylase kinase. In the phosphorylated form, Ca binding affinity of the calmodulin subunit (8) is strongly increased and activation is also possible at low Ca concentrations.

Table 4.6.15 Liver and muscle phosphorylase enzyme activities... 

DiMarco JP, Miles W, Akhtar M, Milstein S, Sharma AD, Platia E, McGovern B, Scheinmman MM, Govier WC (1990) Adenosine for paroxysmal supraventricular tachycardia dose ranging and comparison with verapamil. Assessment in placebo-controlled, multicenter trials. The Adenosine for PSVT Study Group. Ann Intern Med 113(2) 104-110 Dobson JG (1978) Reduction by adenosine of the isoproterenol-induced increase in cyclic 3 , 5 -monophosphate formation and glycogen phosphorylase activity in rat heart muscle. Circ Res 43(5) 785-792... 

The phosphorylase-stimulation assay247 250 is based on the stimulation by branching enzyme of the unprimed reaction in absence of primer activity seen with rabbit muscle phosphorylase a activity. Branching enzyme present in the reaction mixture increases the number of non-reducing ends available to phosphorylase for elongation. 

The activation of skeletal muscle phosphorylase b kinase by cAMP-dependent protein kinase is concomitant with an initial phosphorylation of the /8-subunit [77,78], After 2 mol phosphate per (a/3yS)4 have been incorporated into this subunit, phos-... 

The major substrate of phosphorylase b kinase is phosphorylase b which is phos-phorylated on a single serine residue at position 14, resulting in conversion to the more catalytically active form phosphorylase a [70], Phosphorylation of skeletal muscle phosphorylase also results in conversion of the Mr 200000 dimeric b form to the Mr 400000 tetrameric a form, whereas phosphorylation of the liver enzyme does not alter its dimeric structure [82]. Phosphorylase a is much less dependent than phosphorylase b upon the allosteric activator AMP [82], Since the activity of phosphorylase is rate-limiting for glycogen breakdown, its activation by phosphorylase b kinase results in enhanced glycogenolysis and glucose release from the liver. 

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. 

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