Amino acids in muscles

Figure 8.17 The metabolism of branched-chain amino acids in muscle and the fate of the nitrogen and oxoacids. The a-NH2 group is transferred to form glutamate which is then aminated to form glutamine. The ammonia required for aminab on arises from glutamate via glutamate dehydrogenase, but originally from the transamination of the branded chain amino acids. Hence, they provide both nitrogen atoms for glutamine formation.

Figure 8.24 Some fates of glutamine that is released by muscle. Glutamine is released from the store of glutamine in the muscle but, for immune system and bone marrow, it may also be provided from adipocytes (Chapter 17). It is assumed that glutamine is present as a free amino acid in muscle and that there is a specific transport protein in the plasma membrane that can be regulated.

Turinsky J., and Long, C. L. (1990). Free amino acids in muscle Effect of muscle fiber population and denervation. A i. /. Physiol 258, E4S5-E491. 

Norlen H, Dimberg M, Allgen LG, Vinnars E. Water and electrolytes in muscle tissue and free amino acids in muscle and plasma in connection with transurethral resection of the prostate. II. Isotonic 2.2% glycine solution as an irrigating fluid. Scand J Urol Nephrol 1990 24(2) 95-101. 

Table 2. Concentration of Amino Acids in Muscle Protein Compared with the A-V Plasma Difference Across the Arm and Leg of Fasting and Insulin-Injected Subjects.

However, not all of the functions of other abundant non-standard amino acids are known, for example taurine is a major amino acid in muscle and brain tissues, but although many functions have been proposed, its precise role in the body has not been determined. 

Influence of Endopeptidases and Aminopeptidases on the Production of Taste Peptides and Free Amino Acids in Muscle Foods... 

Not all toxic organophosphoms compounds have uses beneficial to humans. Sarin is an extremely toxic nerve gas that is lethal to humans. In March 1995 this substance was released in a terrorist attack on a Japanese subway, resulting in several deaths and many serious injuries. Sarin and related nerve gases bind an amino acid in the enzyme responsible for muscle action. When this enzyme is deactivated, muscles contract but cannot relax. Even a small dose can be lethal if the nerve gas reaches the muscles of the heart. 

Albumin and amino acid loss Muscle wasting Increased adipose tissue Fibrin formation in dialysate... 

Amino groups released by deamination reactions form ammonium ion (NH " ), which must not escape into the peripheral blood. An elevated concentration of ammonium ion in the blood, hyperammonemia, has toxic effects in the brain (cerebral edema, convulsions, coma, and death). Most tissues add excess nitrogen to the blood as glutamine. Muscle sends nitrogen to the liver as alanine and smaller quantities of other amino acids, in addition to glutamine. Figure I-17-1 summarizes the flow of nitrogen from tissues to either the liver or kidney for excretion. The reactions catalyzed by four major enzymes or classes of enzymes involved in this process are summarized in Table T17-1. 

Table 8.2 Concentrations of free amino acids in plasma, Liver and muscle of humans...

Liver, small intestine, muscle and kidney all participate in amino acid catabolism with the liver, under most conditions, playing the major role, but the metabolism of specific amino acids in the other three tissues is of considerable biochemical and physiological importance (see below). 

The major role of skeletal muscle is movement, which is described and discussed in Chapter 13). Nevertheless, since muscle comprises 40% of the body it is large enough to play a part in control of the blood concentrations of the major fuels glucose, fatty acids, triacylglycerol and some amino acids. Skeletal muscle contains the largest quantity of protein in the body, which is used as a source of amino acids under various conditions (e.g. starvation, trauma, cancer see above). It plays an important part in the metabolism, in particular, of branched-chain amino acids, glutamine and alanine, which are important in the overall metabolism of amino acids in the body (discussed below). 

Figure 7.2 Glucose regulation by insulin. Increased concentrations of glucose and amino acids in the circulation, vagus nerve stimulation and circulating hormones e.g. GLP) stimulate insulin secretion by 3-cells of the pancreatic islets. Insulin suppresses glucose production by the liver and increases glucose uptake by skeletal muscle and fat, leading to decreased circulating glucose concentrations.

Amine build-up in fish muscle usually results from decarboxylation of amino acids in the muscle by enzymes of bacterial origin. This review will present information on the activity of bacterial decarboxylases and the formation of amines in fish. Mechanisms of decarboxylase action and production of bacterial decarboxylases in fish muscle are discussed. Emphasis is placed upon studies dealing with formation of histidine decarboxylase and histamine. Histamine, because of its involvement in Scombroid food poisoning, has been extensively studied with regard to its formation in fish and fishery products. 

The glucocorticoids have important dose-related effects on carbohydrate, protein, and fat metabolism. The same effects are responsible for some of the serious adverse effects associated with their use in therapeutic doses. Glucocorticoids stimulate and are required for gluconeogenesis and glycogen synthesis in the fasting state. They stimulate phosphoenolpyruvate carboxykinase, glucose-6-phosphatase, and glycogen synthase and the release of amino acids in the course of muscle catabolism. 

The net results of these actions are most apparent in the fasting state, when the supply of glucose from gluconeogenesis, the release of amino acids from muscle catabolism, the inhibition of peripheral glucose uptake, and the stimulation of lipolysis all contribute to maintenance of an adequate glucose supply to the brain. 

Reference values for amino acids in the urine show a rather sharp decrease from the neonatal period to adulthood (Table 2.1.6). This is mainly due to the maturation of the renal tubular reabsorption system, but is also the result of increasing muscle mass with age, giving rise to increasing creatinine production. 

What is most remarkable is that cells can produce proteins with strikingly different properties and activities by joining the same 20 amino acids in many different combinations and sequences. From these building blocks different organisms can make such widely diverse products as enzymes, hormones, antibodies, transporters, muscle fibers, the lens protein of the eye, feathers, spider webs, rhinoceros horn, milk proteins, antibiotics, mushroom poisons, and myriad other substances having distinct biological activities (Fig. 3-1). Among these protein products, the enzymes are the most varied and specialized. Virtually all cellular reactions are catalyzed by enzymes. 

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