Alanine release from muscle

Muscle protein catabolism generates amino acids some of which may be oxidized within the muscle. Alanine released from muscle protein or which has been synthesized from pyruvate via transamination, passes into the blood stream and is delivered to the liver. Transamination in the liver converts alanine back into pyruvate which is in turn used to synthesise glucose the glucose is exported to tissues via the blood. This is the glucose-alanine cycle (Figure 7.11). In effect, muscle protein is sacrificed in order to maintain blood adequate glucose concentrations to sustain metabolism of red cells and the central nervous system. 

The alanine released from muscle is taken up by the liver. Glutamate—pyruvate aminotransferase, in liver, catalyzes the conversion of alanine to pyruvate. This pyruvate can then be used for gluconeogenesis. The amino group initially dis-... 

Net breakdown of muscle can occur with either exercise or prolonged fasting. The mechanisms that control the breakdown of the various types of protein found in muscle are not well understood. It has, however, been established that while the branched-chain amino acids (BCAAs) released tend to be oxidized for energy in the muscle cell, other released amino acids enter the bloodstream for catabolism, and perhaps gluconeogenesis, in the liver Examination of the amino acids released from skeletal muscle reveals an apparent anomaly alanine accounts for or ly about 6% of the amino acids of muscle, but for about 35% of the amino acids released from muscle during exerdse. 

During fasting, amino acids are released from muscle protein. Some of the amino acids are partially oxidized in muscle and their remaining carbons are converted to other amino acids. Thus, although about 50% of the amino acids released into the blood from muscle are alanine and glutamine, these two amino acids constitute much less than 50% of the total amino acid residues in muscle protein. 

The alanine released by muscle is also produced by the glucose-alanine cycle, which involves the transport of glucose from the liver to muscle and the return of carbon atoms to the liver as alanine. 

A. Alanine and glutamine are the major amino adds released from muscle. Glutamine is further metabolized in the gut and the kidney. Alanine is the major amino add that is converted to glucose in the liver. 

The fate of the lactate produced is primarily oxidative (55-70%). Lactate released from muscle enters exclusively oxidative tissues in which lactate concentration is low (like heart, diaphragm and brain), is converted back to pyruvate, and is oxidized. Retained lactate is oxidized to generate ATP needed to replenish phosphocreatine stores following exercise and to restore normal distribution of Ca +, Na+, and K+. About 15% is used in gluconeogenesis and subsequent glycogenesis. The balance is converted to alanine, glutamate, or other substances. 

Amino acids Amino acids are derived from proteolysis in muscle. All except leucine and lysine are potentially gluconeogenic in mammals however, alanine is the primary amino acid released from muscle. This alanine is derived from muscle protein and from the metabolism of many other amino acids. 

Some of the alanine released from skeletal muscle is derived directly from protein degradation. The carbon skeletons of valine, isoleucine, aspartate, and glutamate, which are converted to malate and oxaloacetate in the TCA cycle, can be converted to pyruvate and subsequently transaminated to alanine. The extent to which these amino acids contribute carbon to alanine efflux differs between different types of muscles in the human. These amino acids also may contribute to alanine efflux from the gut. 

However, the table shows that, in contrast to the relative concentrations of amino acids found in muscle protein, a large part of this released amino acid N is made up of alanine and glutamine (not shown in the table). It can be concluded that much of the amino acid pool in muscle becomes transaminated to these two amino acids. These two amino acids released from muscle are then quantitatively taken up by the liver where the alanine provides a source of carbon for gluconeogenesis and both alanine and glutamine are donors of NH2 for urea synthesis (Fig. 4). When insulin... 

Parry-Billings M, Bevan SJ, Opara E, Newsholme EA. Effects of changes in cell volume on the rates of glutamine and alanine release from rat skeletal muscle in vitro. BiochemJ 1991-,216 559-561. 

Hormones can modify the concentration of precursors, particularly the lipolytic hormones (growth hormone, glucagon, adrenaline) and cortisol. The lipolytic hormones stimulate lipolysis in adipose tissue so that they increase glycerol release and the glycerol is then available for gluconeogenesis. Cortisol increases protein degradation in muscle, which increases the release of amino acids (especially glutamine and alanine) from muscle (Chapter 18). 

For each alanine molecule formed from proline in this pathway, 12 molecules of ATP are generated from ADR What happens to the alanine It is released from the muscle into the... 

Figure 8.25 Excess ammonia in the muscle is used to form alanine. Ammonia is released from several reactions and is incorporated into alanine via glutamate dehydrogenase and transamination. OG - oxoglutarate. Alanine is released into the blood from volece it is removed by the liver.

Nitrogen is transported from muscle to the liver in two principal transport forms. Glutamate is formed by transamination reactions, but the nitrogen is then transferred to pyruvate to form alanine, which is released into the blood (Figure 23.15). 

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