Protein breakdown

A substrate is a substance that is the basic component of an organism. Protein substrates are amino acids, which are essential to life Protein substrates are amino acid preparations that act to promote the production of proteins (anabolism). Amino acids are necessary to promote synthesis of structural components, reduce the rate of protein breakdown (catabolism), promote wound healing, and act as buffers in the extracellular and intracellular fluids. Crystalline amino acid preparations are hypertonic solutions of balanced essential and nonessential amino acid concentrations that provide substrates for protein synthesis or act to conserve existing body protein. 

Amino acids promote the production of proteins, enhance tissue repair and wound healing, and reduce the rate of protein breakdown. Amino acids are used in certain disease states, such as severe kidney and liver disease, as well as in TPN solutions. (See the last section of this chapter for a more detailed discussion of TPN.) TPN may be used in patients with conditions such as impairment of gastrointestinal absorption of protein, in patients with an increased requirement for protein, as seen in those with extensive bums or infections, and in patients with no available oral route for nutritional intake ... 

Ketteihut, I.C. Goldberg, A.L (1988). Tumor necrosis factor can induce fever in rats without activating protein breakdown in muscle or lipolysis in adipose tissue. J. Clin. Invest. 81, 1384-1389. 

The smdy of tissue protein breakdown in vivo is difficult, because amino acids released during intracellular breakdown of proteins can be extensively reutilized for protein synthesis within the cell, or the amino acids may be transported to other organs where they enter anabohc pathways. However, actin and myosin are methylated by a posttranslational reaction, forming d-methylliistidine. During intracellular breakdown of actin and myosin, 3-methylhistidine is released and excreted into the urine. The urinary output of the methylated amino acid provides a rehable index of the rate of myofibrillar protein breakdown in the musculature of human subjects. 

Dextrose used in PN compounding typically is provided as a 70% stock solution (70 g/100 mL), although some institutions use a 50% stock solution. The final dextrose concentration in the PN solution typically should not exceed 35%. Hydrous dextrose provides 3.4 kcal/g (14.2 kj/g). A dextrose infusion rate of 2 mg/kg per minute should be sufficient to suppress gluconeogenesis and prevent protein breakdown in adults.5 Continuous dextrose infusion rate in adult patients generally should not exceed 4 to 5 mg/kg per minute in most hospitalized patients.6,7... 

Glauman H, Ballard J. Lysosomes and Their Role in Protein Breakdown, Academic Press, New York, 1987. 

The production of both anabolic sex hormones and growth hormone decreases in aging and the result is muscle loss. Also the loss of neural motor cells gives rise to atrophy of the muscles. In the aging process an increased inflammatory activity can be seen and it causes degeneration of muscle tissue through insuline resistance and activation of protein breakdown enzymes. The increased inflammatory activity is due to both normal aging as well as the presence of multiple chronic diseases (Dutta 1997. 

A. J. Barrett, Introduction The Classification of Proteinases , in Protein Breakdown in Health and Diseases , Ciba Foundation Symposium 75, Eds. D. Evered, J. Whelan, Ex-cerpta Medica, Amsterdam, 1980, p. 1-13. 

A. Hershko, H. Heller, E. Eytan, G. Kakklij, I. A. Rose, Role of the Alpha-Amino Group of Protein in Ubiquitin-Mediated Protein Breakdown , Proc. Natl. Acad. Sci. U.S.A. 1984, 81, 7021-025. 

L. Marzella, H. Glaumann, Autophagy, Microautophagy and Crinophagy as Mechanisms for Protein Degradation , in Lysosomes Their Role in Protein Breakdown , Eds. H. Glaumann, F. J. Ballard, Academic Press, London, 1987, p. 317-367. 

Heeshko, A. and A. Ciechanovee, Mechanisms of intracellular protein breakdown. Annu Rev Biochem, 1982, 51, 335-64. 

I. A. Proposed role of ATP in protein breakdown conjugation of proteins with multiple chains of the polypeptide of ATP-dependent proteolysis. 

Heeshko, a., Hellee, H., Elias, S. and CiECHANOVEE, A. Components of ubiquitin-protein ligase system. Resolution, affinity purification, and role in protein breakdown. J Biol Chem 1983, 258, 8206-14. 

P., and Goldberg, A. L. An archaebacterial ATPase, homologous to ATPases in the eukaryotic 26 S proteasome, activates protein breakdown by 20 S proteasomes. J. Biol. Chem. 1999, 274, 26008-26014. 

When older proteins are broken down in the body, they must be replaced. This concept is called protein turnover, and different types of proteins have very different turnover rates. Protein synthesis occurs during the process of translation on ribosomes. Protein breakdown occurs generally in two cellular locations ... 

This system is quantitatively the most important process for protein breakdown in mammalian cells. It is so named because it involves the proteolytic enzyme, the protea-some, and the peptide ubiquitin. 

In 1987 the gene responsible for muscular dystrophy was identified, leading to the isolation of a protein, known as dystrophin, which is either totally absent in Duchenne, or partially absent in the Becker type. The protein is located on the inside of the plasma membrane of all muscles (and some neurones). Although its precise function is not known, the mutant form results in structural abnormalities of the plasma member which results in degradation of myofibrils, but the hnk between the abnormalities of the membrane and degradation is not known. One theory is that it leads to an increase in the activity of a Ca " ion channel in the membrane and, therefore, a marked increase in the Ca ion concentration in the cytosol. This chronic elevation results in the activation of calpain, which leads to protein breakdown and the degeneration within the fibre (Chapter 13). 

Glycogen synthesis in liver and muscle Glycogen breakdown in liver Fat synthesis in liver Fat breakdown in adipose tissue Protein synthesis in muscle Protein breakdown in muscle... 

The amino acids that are made available as a result of protein degradation in starvation are nsed as precursors of glucose, which is required for the brain. The decline in starvation-induced protein degradation is a result of the decreased requirement for glucose by the brain due to the increase in utilisation of ketone bodies. The qnestion arises, therefore, as to the mechanism by which the protein breakdown in muscle is reduced. Two answers, which are interdependent, have been put forward (i) decreased metabolic activity in tissues, and (ii) a decrease in the plasma level of thyroxine and hence triiodothyronine. 

Dnring starvation there is a decrease in the resting energy expenditnre which is due to reduced metabolic activity in most tissnes including, perhaps surprisingly, the brain. The decrease in muscle protein breakdown may, therefore, be a simple consequence of a decrease in the rate of glucose oxidation, so that gluconeogenesis from the amino acids is decreased. 

Stimulation of muscle protein breakdown Glucocorticoids Interleukin-1... 

The process for sheepskin is rather more gentle in order to avoid damage to top quiity wool that can be spun into valuable wool yarn. Liquid protease preparations such as pancreatic extracts of trypsin and chymotrypsin are applied to the flesh side of the sheepskin. The proteases diffuse to the basal papilla of the hair follicle, and subsequent protein breakdown releases the wool hair with minimal damage 34),... 

In muscle, the extensive glycogen reserves are exclusively used for the muscles own requirements (see p. 320). The slowly initiated protein breakdown in muscle supplies amino acids for gluconeogenesis in the liver. 

Hershko, A., Ciechanover, A., Heller, H., Haas, A. L. and Rose, 1. A. (1980). Proposed role of ATP in protein breakdown Conjugation of proteins with multiple chains of the polypeptide of ATP-dependent proteolysis. Proc.Natl. Acad. Sci. USA 77, 1783-1786. 

The wall of the small intestine is permeable to water and to small molecules such as the amino acids produced by protein breakdown and sugars produced by carbohydrate breakdown so this system is a reactor-separator combination, a membrane reactor. Finally the undigested food passes into the large intestine, where more water is removed through the permeable wall before exiting the reactor. 

Gortisol promotes net protein breakdown in skeletal muscle to provide amino acids as precursors for gluconeogenesis and ketone body synthesis (keto-genesis). 

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