Endogenous muscle proteins

Figure 8.13 The central role of transdeamination in metabolism of amino adds and further metabolism of the oxoacids in the liver. The box contains the reactions for conversion of the amino acids to their respective oxoacids. Processes are as follows (1) digestion of protein in the intestine and absorption of resultant amino acids, (2) degradation of endogenous protein to amino acids (primarily but not exclusively muscle protein), (3) protein synthesis, (4) conversion of amino acid to other nitrogen-containing compounds (see Table 8.4), (5) oxidation to CO2, (6) conversion to glucose via gluconeogenesis, (7) conversion to fat.

Many studies have described how L-tryptophan itself may be an important regulator of hepatic protein synthesis under normal or abnormal conditions.14 16 Based upon these reports, the hypothesis of Gessa et al.3 that the primary effect of administering a tryptophan-free amino acid mixture that lowers endogenous tryptophan serum levels via increased incorporation of tryptophan into hepatic and other proteins must be viewed with caution. Possibly, the overall decrease in serum tryptophan that occurs is due to decreased ingestion of tryptophan along with adequate intake of other amino acids which may stimulate muscle protein catabolism. This may liberate tryptophan and other amino acids into the circulation, which may rapidly enhance hepatic protein synthesis and thereby deplete plasma tryptophan levels. This mechanism was proposed earlier in studies with dietary tryptophan deficiency.17... 

Meat Proteins. The conversion of muscle to meat is characterized by a complex series of biochemical reactions largely involving endogenous enzymes. The ultimate objective of the meat processor is to produce an aged, tender piece of meat. However, before this can happen there occurs a progression of biochemical reactions that converts the muscle tissue to meat. In many cases, the enzymic reactions necessary for the formation of meat are poorly understood. For a detailed discussion of the biochemical reactions in meat chemistry the reader is referred to texts by Lawrie (31) and Price and Schweigert (32). The purpose of the present discussion is to pinpoint a few of the enzymic reactions which alter the functionality of muscle proteins. 

Stanley (1983) concluded that postmortem events influence the physical properties of meat, not only through rigor mortis, but also as a result of the action of numerous endogenous enzymes on myofibrillar structure, and perhaps, connective tissue as well. A major structural alteration that has been observed in postmortem muscle is Z-disc degradation. The unreactive chemical nature of collagen may preclude any major attack by endogenous muscle enzymes on this fibrous protein (Offer et al, 1988). 

ADP-Ribosyltransferase and Endogenous Acceptor Proteins in Animal Muscle Tissues ADP-Ribosylation of Ca2+-Dependent ATPase in Rabbit Skeletal Muscle Sarcoplasmic Reticulum and the Effect of Basic Peptides... 

For example, intense proteolyses of muscle proteins, due to the action of endogenous proteolytic enzymes, have been reported to occur during the processing of dry-cured ham. This gives rise to the formation of free amino acids and short peptides (especially from actin through the action of cathepsin D in meat and from caseines as a result of plasmin and other proteases in cheeses) that contribute directly or indirectly to the flavour characteristics of the final product. In the case of octapeptide Lys-Gly-Asp-Glu-Glu-Ser-Leu-Ala, isolated from beef broth, this reportedly showed umami taste with a threshold value of about 500 mg/1. 

Casnellie, J. E., and Greengard, P., 1974, Guanosine 3, 5 -cyclic monophosphate dependent phosphorylation of endogenous substrate proteins in membranes of mammalian smooth muscle, Proc. Natl. Acad. Sci. USA 71 1891. 

The skeletal muscle Ca channels also can be phosphorylated in vitro by a protein kinase endogenous to the T-tubule membranes [111,115]. This kinase is neither Ca - nor cyclic nucleotide-dependent [115], and is interesting in that it phosphorylates primarily the P subunit while the ai subunit is a poor substrate. However, the amount of this kinase that co-purifies with the T-tubule membranes is variable, and consequently, very few studies have been performed. So far, only low levels of phosphorylation have been obtained (no more than 0.2 mol phosphate/ mol P subunit) and no functional effects of this phosphorylation have been observed in reconstitution studies. 

The chemical transmitters may be small molecules— notably acetylcholine, norepinephrine, epinephrine, serotonin, dopamine, or histamine. Acetylcholine and norpeinephrine are the dominant neurotransmitters in the parasympathetic and sympathetic nervous systems, respectively. Dopamine and serotonin are employed primarily in the central nervous system. Neurotransmitters may also be more complex peptides (small proteins) such as substance P, vasopressin, endorphins, and enkephalins. The latter agents are of particular importance to our considerations of opium since they represent the endogenous opiates—agents that exist within the body whose actions are mimicked by exogenous, or outside, agents such as morphine, heroin, codeine, and so on. These neurotransmitters serve to convey information between neurons across the synaptic cleft (the junction where two neurons meet) or at the neuroeffector junction (the site between neuron and an innervated organ such as muscle or secretory gland). 

Acetylcholine (ACh) is an example of an endogenous neurotransmitter that binds to more than one receptor type, the nicotinic acetylcholine receptor (nAChR) which preferentially binds nicotine and the muscarinic receptor which binds muscarine, a mushroom alkaloid. The latter is a G protein-coupled receptor while the nACh receptor is an excitatory ligand-gated ion channel that transports Na-i- ions. Nicotinic cholinergic receptors are found in the CNS, autonomic ganglia, and at the neuromuscular junction of skeletal muscles. They are a possible target for anaesthetics. 

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