Vitamin glycogen phosphorylase

Six compounds have vitamin Bg activity (Figure 45-12) pyridoxine, pyridoxal, pyridoxamine, and their b -phosphates. The active coenzyme is pyridoxal 5 -phos-phate. Approximately 80% of the body s total vitamin Bg is present as pyridoxal phosphate in muscle, mostly associated with glycogen phosphorylase. This is not available in Bg deficiency but is released in starvation, when glycogen reserves become depleted, and is then available, especially in liver and kidney, to meet increased requirement for gluconeogenesis from amino acids. 

Pyridoxal phosphate is a coenzyme for many enzymes involved in amino acid metabolism, especially in transamination and decarboxylation. It is also the cofactor of glycogen phosphorylase, where the phosphate group is catalytically important. In addition, vitamin Bg is important in steroid hormone action where it removes the hormone-receptor complex from DNA binding, terminating the action of the hormones. In vitamin Bg deficiency, this results in increased sensitivity to the actions of low concentrations of estrogens, androgens, cortisol, and vitamin D. 

As indicated in Section 6.3.3 and Table 6.2 the key control step is mediated by glycogen phosphorylase, a homodimeric enzyme which requires vitamin B6 (pyridoxal phosphate) for maximum activity, and like glycogen synthase (Section 6.2) is subject to both allosteric modulation and covalent modification. 

Muscle glycogen phosphorylase is one of the most well studied enzymes and was also one of the first enzymes discovered to be controlled by reversible phosphorylation (by E.G. Krebs and E. Fischer in 1956). Phosphorylase is also controlled allosterically by ATP, AMP, glucose and glucose-6-phosphate. Structurally, muscle glycogen phosphorylase is similar to its hepatic isoenzyme counterpart composed of identical subunits each with a molecular mass of approximately 110 kDa. To achieve full activity, the enzyme requires the binding of one molecule of pyridoxal phosphate, the active form of vitamin B6, to each subunit. 

Pyridoxine is present in food in the free form and as a glucoside, which may undergo partial hydrolysis in the gut lumen, or may be absorbed intact. Although pyridoxine is associated with the enzyme glycogen phosphorylase in muscles, it is not released in response to a dietary deficiency therefore it cannot be regarded as a storage form of the vitamin. 

Glycogen phosphorylases belong to the group of vitamin B6 enzymes bearing a catalytic mechanism that involves the participation of the phosphate group of pyridoxal-5 -phosphate (FTP). The proposed mechanism is a concerted one with a front-side attack, as can be seen in Fig. 5 [109]. In the forward direction, e.g.. 

All aminotransferases have the same prosthetic group and the same reaction mechanism. The prosthetic group is pyridoxal phosphate (PLP), the coenzyme form of pyridoxine, or vitamin B6. We encountered pyridoxal phosphate in Chapter 15, as a coenzyme in the glycogen phosphorylase reaction, but its role in that reaction is not representative of its usual coenzyme function. Its primary role in cells is in the metabolism of molecules with amino groups. 

There may be multiple metabolicpoolsofthe vitamin, with very different rates of turnover. In this case, short-term and long-term smdies will give very different estimates of the fractional rate of turnover of the total body pool. As discussed in Section 9.6.1, this is known to be a problem with vitamin Be, because some 80% of the total body pool is associated with muscle glycogen phosphorylase and has a much lower fractional turnover rate than the remaining 20%. 

Vitamin Be has a central role in the metabolism of amino acids in transaminase reactions (and hence the interconversion and catabolism of amino acids and the synthesis of nonessential amino acids), in decarboxylation to yield biologically active amines, and in a variety of elimination and replacement reactions. It is also the cofactor for glycogen phosphorylase and a variety of other enzymes. In addition, pyridoxal phosphate, the metabolically active vitamer, has a role in the modulation of steroid hormone action and the regulation of gene expression. 

Some 80% of the body s total vitamin Be is as pyridoxal phosphate in muscle, and some 80% of this is associated with glycogen phosphorylase. This does not seem to function as a reserve of the vitamin and is not released from the muscle in deficiency. 

Muscle pyridoxal phosphate is released into the circulation (as pyridoxal) in starvation as muscle glycogen reserves are exhausted and there is less requirement for glycogen phosphorylase activity. Under these conditions, it is potentially available for redistribution to other tissues, especially the liver and kidneys, to meet the increased requirement for gluconeogenesis from amino acids (Black et al., 1978). However, during both starvation and prolonged bed rest, there is a considerable increase in urinary excretion of 4-pyridoxic acid, suggesting that much of the vitamin Be released as a result of depletion of muscle glycogen and atrophy of muscle is not redistributed, but rather is ca-tabolized and excreted (Cobum et al., 1995). 

Oka T, Komori N, Kuwahata M, Suzuki I, Okada M, and Natori Y (1994) Effect of vitamin Be deficiency on the expression of glycogen phosphorylase mRNA in rat liver and skeletal muscle. Experientia 50, 127-9. 

The normed muscle concentration of pyridoxal phosphate is of the order of 10 nmol per g in patients with McArdle s disease (glycogen storage disease from congenital lack of glycogen phosphorylase), the muscle content of pyridoxal phosphate is reduced to one-fifth of this. There is some evidence that patients with McArdle s disease show signs of vitamin Be deficiency, su esting that the muscle pool of the vitamin is important in maintenance of vitamin Be homeostasis (Beynon et ed., 1995). 

It acts as a cofactor for glycogen phosphorylase in glycogenolysis. Decreased glucose tolerance may be associated with vitamin B-6 deficiency. 

This large, slow turnover pool is most likely in muscle, which contains about 70-80% of the vitamin B6 in the body (Cobum et al, 1985,1988b). The muscle pool is associated primarily with glycogen phosphorylase (Butler et al, 1985). Using pyridoxal phosphate as an indicator of glycogen phospho-... 

A third consequence is the question of whether glycogen phosphorylase conserves vitamin B6 because it has a very low rate of degradation or because it recycles the pyridoxal phosphate very efficiently. Based on the rapid decay of the free vitamin B6 pool in muscle, the slow turnover of the protein-bound vitamin B6 pool, the failure to detect apo-phosphorylase, and the agreement between turnover rates based on amino acid or vitamin B6 data, Beynon et al. (1986) concluded that recycling would be minimal. We feel that the effect of vitamin B6 intake on turnover rates tends to favor the recycling option under conditions of limited intake. However, more data are needed before a final decision can be made. 

FIG. 1. Interrelationships between vitamin B6 and phosphorylase metabolism. The low rate of turnover of glycogen phosphorylase (gpb) and the lack of exchange of free and protein-bound PLP mean that exchange into the muscle pool is largely controlled by the kinetics of turnover of the enzyme. At present, it is not known whether resolution of the holo-enzyme is a prerequisite or consequence of phosphorylase degradation. Reproduced with permission of llie Biochemical Journal. 

If the phosphorylase pool plays an important part in vitamin B6 kinetics, it might be anticipated that this metabolism would be disturbed in patients suffering from MciVdle s disease, a rare metabolic myopathy caused by an absence of functional muscle glycogen phosphorylase. The absence of this enzyme means that patients cannot break down their muscle glycogen reserves. Other energy sources within the muscle are rapidly depleted... 

Beynon, R. J., leyland, D. M., Evershed, R. P., Edwards, R. H. T., and Cobum, S. P. (1996). Measurement of the turnover of glycogen phosphorylase by gas chromatography/mass spectrometry using stable isotope derivatives of pyridoxine (Vitamin B6). Biochem. J. 317, 613-619. 

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