Fasting protein degradation

D. Skeletal muscle in its resting state can satisfy most of its energy needs by oxidation of fatty acids taken up from blood, and during the early stages of fasting, protein degradation in the muscle is increased. 

The plasma ketone body level increases twice as fast over live days starvation in lean snbjects compared with obese this may be an attempt by the body to restrict protein degradation in the lean by providing an alternative fnel to glucose for the brain as qnickly as possible. 

Ruminants. Aldrich and Merchen (27) studied the effect of heat treatment of whole soybean on protein digestion by mminants. They reported that increasing the extmsion temperature from 220°F to 320°F at 20°F increments resulted in a linear decrease of in situ degradation of soybean protein. As expected, raw soybean protein degraded very fast. The extent of degradation was 84.1%, 45.7%, 40%, 40.9%, 36.7%, and 30.4% for the raw, 220°F, 240°F, 260°F, 280°F, 300°F, and 320°F treatments, respectively. In other words, extmded soybean at 320°F had a bypass protein value of 69.6% as compared with 15.9% for raw soybeans. 

Fasting (- Insulin) Low ratio —> Protein Degradation / Metabolism. Roles of Protein Degradation/ Metabolism ... 

During fasting or infective stress, protein degradation in skeletal muscle is activated to increase the supply of amino acids in the blood for glu-coneogenesis, or for the synthesis of antibodies and other component of the immune response. Under these conditions, synthesis of ubiquitin, a protein that targets proteins for degradation in proteosomes, is increased by the steroid hormone cortisol. 

The brain is glucose dependent, but, like many cells in the body, can use BCAA for energy. The BCAA also provide a source of nitrogen for neurotransmitter synthesis during fasting. Other amino acids released from skeletal muscle protein degradation also serve as precursors of neurotransmitters. 

In these hypercatabolic states, skeletal muscle protein synthesis decreases, and protein degradation increases. Oxidation of BCAA is increased and glutamine production enhanced. Amino acid uptake is diminished. Cortisol is the major hormonal mediator of these responses, although certain cytokines may also have direct effects on skeletal muscle metabohsm. As occurs during fasting and metabolic acidosis, increased levels of cortisol stimulate ubiquitin-mediated proteolysis, induce the synthesis of glutamine synthetase, and enhance release of amino acids and glutamine from the muscle cells. 

Saundersoa C.L. Leslie, S. (1988). Muscle growth and protein degradation during early development in chicks of fast and slow growing strains. Comp. Biochem. Physiol, 89A, 333-7. 

The alanine cycle accomplishes the same thing as the Cori cycle, except with an add-on feature (Fig. 17-11). Under conditions under which muscle is degrading protein (fasting, starvation, exhaustion), muscle must get rid of excess carbon waste (lactate and pyruvate) but also nitrogen waste from the metabolism of amino acids. Muscle (and other tissues) removes amino groups from amino acids by transamination with a 2-keto acid such as pyruvate (oxaloacetate is the other common 2-keto acid). 

N-terminal sequencing is normally undertaken by Edman degradation (Figure 7.5). Although this technique was developed in the 1950s, advances in analytical methodologies now facilitate fast and automated determination of up to the first 100 amino acids from the N-terminus of most proteins, and usually requires a sample size of less than 1 umol to do so (Figure 7.6). 

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