Purine nucleotide cycle

Deficiency of the muscle-specific myoadenylate deaminase (MADA) is a frequent cause of exercise-related myopathy and is thought to be the most common cause of metabolic myopathy. MADA catalyzes the deamination of AMP to IMP in skeletal muscle and is critical in the purine nucleotide cycle. It is estimated that about 1-2% of all muscle biopsies submitted to medical centers for pathologic examination are deficient in AMP deaminase enzyme activity. MADA is 10 times higher in skeletal muscle than in any other tissue. Increase in plasma ammonia (relative to lactate) after ischemic exercise of the forearm may be low in this disorder, which is a useful clinical diagnostic test in patients with exercise-induced myalgia... 

The Purine Nucleotide Cycle and Salvage Pathways for Purines... 

Muscular work is accompanied by the production of ammonia, the immediate source of which is adenosine 5 -phosphate (AMP).301 302 This fact led to the recognition of another substrate cycle (Chapter 11) that functions by virtue of the presence of a biosynthetic pathway and of a degradative enzyme in the same cells (cycle A, Fig. 25-17). This purine nucleotide cycle operates in the brain303 304 as well as in muscle. The key enzyme 5-AMP aminohydrolase (AMP deaminase step a, Fig. 25-17) also occurs in erythrocytes and many other tissues.304 305 Persons having normal erythrocyte levels but an absence of this enzyme in muscles suffer from muscular weakness and cramping after exercise.306... 

Muscle can metabolize AMP by using the purine nucleotide cycle. The initial step in this cycle, catalyzed by AMP deaminase, is the conversion of AMP into IMP. 

The purine nucleotide cycle serves to release NH4+ from amino acids, particularly in muscle. 

Aspartate is converted to fumarate via reactions of the urea cycle and the purine nucleotide cycle. 

The purine nucleotide cycle also is involved in muscle energy production. During intense stimulation, or when O2 supply is limited, the high-energy bond of ADP is used to synthesize ATP via the myokinase reaction (Figure 21-12). The resulting AMP can dephosphorylate to adenosine, which diffuses out of the cell. Conversion of AMP to IMP via adenylate deaminase and then to adenylosuccinate helps sustain the myokinase reaction, especially in FG fibers, by reducing accumulation of AMP. It may also reduce the loss of adenosine from the cell, since nucleosides permeate cell membranes while nucleotides do not. 

Ammonia is produced by oxidative and nonoxidative deaminations catalyzed by glutaminase and glutamate dehydrogenase (Chapter 17). Ammonia is also released in the purine nucleotide cycle. This cycle is prominent in skeletal muscle and kidney. Aspartate formed via transamination donates its a-amino group in the formation of AMP the amino group is released as ammonia by the formation of IMP. 

In muscle, a unique nucleotide reutilization pathway, known as the purine nucleotide cycle, uses three enzymes myoadenylate deaminase, adenylosuccinate synthetase, and adenylosuccinate lyase. In this cycle, AMP is converted to IMP with formation of NH3, and IMP is then reconverted to AMP. Myoadenylate deaminase deficiency produces a relatively benign disorder of muscle... 

Myoadenylate deaminase (or AMP deaminase) deficiency is a relatively benign muscle disorder characterized by fatigue and exercise-induced muscle aches. This disorder is presumably inherited as an autosomal recessive trait. The relationship between the exercise-induced skeletal muscle dysfunction and AMP deaminase deficiency is explained by an interruption of the purine nucleotide cycle. 

The purine nucleotide cycle of muscle consists of the conversion of AMP —> IMP AMP and requires AMP deaminase, adenylosuccinate synthetase, and adenylosuccinate lyase (Figure 27-24). Flux through this cycle increases during exercise. Several mechanisms have been proposed to explain how the increase in flux is responsible for the maintenance of appropriate energy levels during exercise (Chapter 21). 

Fig. 38.5. Summary of the sources of NH4 for the urea cycle. All of the reactions are irreversible except glutamate dehydrogenase (GDH). Only the dehydratase reactions, which produce NH4 from serine and threonine, require pyridoxal phosphate as a cofactor. The reactions that are not shown occurring in the muscle or the gut can all occur in the liver, where the NH4 generated can be converted to urea. The purine nucleotide cycle of the brain and muscle is further described in Chapter 41.

In muscle and brain, but not in liver, the purine nucleotide cycle allows Nlij to be released from amino acids (see Fig. 38.5). Nitrogen is collected by glutamate from other amino acids by means of transamination reactions. Glutamate then transfers its amino group to oxaloacetate to form aspartate, which supplies nitrogen to the purine nucleotide cycle (see Chapter 41). The reactions of the cycle release fumarate and NH4. The ammonium ion formed can leave the muscle in the form of glutamine. 

Although the major route for aspartate degradation involves its conversion to oxaloacetate, carbons from aspartate can form fumarate in the urea cycle (see Chapter 38). This reaction generates cytosolic fumarate, which must be converted to malate (using cytoplasmic fumarase) for transport into the mitochondria for oxidative or anaplerotic purposes. An analogous sequence of reactions occurs in the purine nucleotide cycle. Aspartate reacts with inosine monophosphate (IMP) to... 

A portion of the salvage pathway that is important in muscle is the purine nucleotide cycle (Fig. 41.13). The net effect of these reactions is the deamination of aspartate to fumarate (as AMP is synthesized from IMP and then deaminated back to IMP by AMP deaminase). Under conditions in which the muscle must generate energy, the fumarate derived from the purine nucleotide cycle is used anapleroti-cally to replenish TCA cycle intermediates and to allow the cycle to operate at a high speed. Deficiencies in enzymes of this cycle lead to muscle fatigue during exercise. 

Fig. 41.13. The purine nucleotide cycle. Using a combination of biosynthetic and salvage enzymes, the net effect is the conversion of aspartate to fumarate plus ammonia, with the fumarate playing an anaplerotic role in the muscle.

In the purine nucleotide cycle (Fig. 42.11), the deamination of AMP to IMP releases NITj. AMP is resynthesized with amino groups provided from aspartate. The aspartate amino groups can arise from the BCAA through transamination reactions. The fumarate can be used to replenish TCA cycle intermediates. 

Fig. 42.11. Purine nucleotide cycle. In skeletal muscle, the purine nucleotide cycle can convert the amino groups of the BCAA to NH3, which is incorporated into glutamine. The compounds containing the amino group released in the purine nucleotide cycle are shown in blue.

Exercise increases the activity of the purine nucleotide cycle, which converts aspartate to fumarate plus ammonia (see Fig. 41.13). The ammonia is used to buffer the proton production and lactate production from glycolysis, and the fumarate is recycled and can form glutamine. 

Tornheim, K. J.M. Lowenstein. 1975. The purine nucleotide cycle. V. Control of phosphofructokinase and glycolytic oscillations in muscle extracts. J. Biol. Chem. 250 6304-14. 

Reichmann, H. DeVivo, D.C. (1991) Comp. Biochem. Physiol. 98R 327-331. Coordinate enzymatic activity of beta-oxidation and purine nucleotide cycle in a diversity of muscle and other organs of rat. Melde, K., Jackson, S., Bartlett, K., Sherratt, H.S.A. Ghisla, S. 99 )Biochem. J. 274,395-400. Metabolic consequences of methylenecyclopropylglycine poisoning in rats. 

Adenylosuccinate formed by adenylosuccinate synthetase is cleaved by adenylosuccinate lyase to form AMP. The reaction steps are illustrated in Fig. 1. Included in the sequence is the additional reaction catalyzed by AMP deaminase. These three enzymes have been suggested to function in a cyclic process termed the purine nucleotide cycle 7,8). The two-step conversion of IMP to AMP is very similar to both the conversion of citrulline to arginine, which involves formation of argininosuccinate as an intermediate, and formation of 5-amino-imidazole 4-carboxamide ribonucleotide from 5-aminoimidazole 4-carboxylate ribonucleotide as part of IMP biosynthesis. Adenylosuccinate lyase is a dual function enzyme catalyzing the cleavage of both adenylosuccinate and 5-aminoimidazole 4-N-succinocarboxamide ribonucleotide. 

Reactions in which aspartate functions as a nitrogen donor have not been studied in the detail that reactions involving glutamine have. The important role of adenylosuccinate synthetase at a branch point of purine metabolism and as a component of the purine nucleotide cycle makes this enzyme a challenging subject for study. This article will deal with regulatory, kinetic, and genetic aspects of adenylosuccinate synthetase from a variety of systems. 

The regulation of mammalian adenylosuccinate synthetase is complicated. It is dependent on the isozyme content and levels in a given tissue as well as the effects of substrate and product levels. The two isozymes may have different metabolic roles either in AMP biosynthesis and interconversion, or in the functions of the purine nucleotide cycle. Most studies have considered kinetic parameters for the isolated enzyme and in only a few instances has regulation been studied in vivo. Sufficient information is available concerning the regulation of the basic isozyme in muscle to consider that enzyme in detail. Factors controlling the acidic isozyme are less clearly defined. 

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