Starvation protein degradation during

Figure 16.11 Pattern of fuel utilisation during prolonged starvation. The major metabolic change during this period is that the rates of ketone body formation and their utilisation by the brain increases, indicated by the increased thickness of lines and arrows. Since less glucose is required by the brain, gluconeogenesis from amino acids is reduced so that protein degradation in muscle is decreased. Note thin line compared to that in Figure 16.9.

Convincing evidence was obtained that protein degradation in E. coli during amino acid starvation depends on the ATP-dependent proteases Lon and Clp (Kuroda et al, 2001). Mutations in Lon and Clp proteases produced the same phenotype as ppkl mutation - the... 

Glucocorticoids released during stress or starvation, oppose the effects of insulin and result in protein degradation (metabolism). 

How long can a steady state of protein degradation be maintained during starvation ... 

Starvation of animals increases the rate of protein degradation in liver. 81,82) and muscle 83-85) and causes about twofold increase of cathepsin D and A within 24 hours in the liver 55, 56). However, increases of cathepsin B and L are much less than that of catheptsin D (55). In the livers of protein-depleted rats, cathepsins B, L, and D decrease during the course of protein depletion 49,81), their activities being one-third of that of controls (on 20% casein diet) after protein depletion for 10 days. It is, however, not known whether changes of proteinase activity are associated with changes in protein synthesis. 

Lowell, B.B., Ruderman, N.B., and Goodman, M.N., Regulation of myofibrillar protein degradation in rat skeletal muscle during brief and prolonged starvation. Metabolism, 35, 1121, 1986. 

During periods of starvation, the brain after a certain time acquires the ability to use ketone bodies (see p. 312) in addition to glucose to form ATP. In the first weeks of a starvation period, there is a strong increase in the activities of the enzymes required for this in the brain. The degradation of ketone bodies in the CNS saves glucose and thereby reduces the breakdown of muscle protein that maintains gluconeogenesis in the liver during starvation. After a few weeks, the extent of muscle breakdown therefore declines to one-third of the initial value. 

In addition to being synthesized or produced by the hydrolysis of dietary protein, amino acids can come from hydrolysis of tissue proteins, e.g., intestinal mucosa or, during starvation, muscle. Amino acids are used in protein synthesis (Chap. 17) they also enter gluconeogenesis and lipogenesis are degraded to provide energy and are used for synthesizing compounds such as purines, pyrimidines, porphyrins, epinephrine and creatine. 

Proteolysis also provides carbon skeletons for gluconeogenesis. During starvation, degraded proteins are not replenished and serve as carbon sources for glucose synthesis. Initial sources of protein are those that turn over rapidly, such as proteins of the intestinal epithelium and the secretions of the pancreas. Proteolysis of muscle protein provides some of three-carbon precursors of glucose. However, survival for most animals depends on being able to move rapidly, which requires a large muscle mass, and so muscle loss must be minimized. 

Skeletal muscle is specialized to perform intermittent mechanical work. As described previously, the energy sources that provide ATP for muscle contraction depend on the degree of muscular activity and the physical status of the individual. During fasting and prolonged starvation, some skeletal muscle protein is degraded to provide amino acids (e.g., alanine) to the liver for gluconeogenesis. 

The major nitrogen-containing metabolites are urea (-84% of total nitrogen), uric acid (-2%), creatinine (-5%), ammonia (-5%), amino acids (-4%), protein (trace), and degradation products of porphyrins (trace, a few tens of mg). The amounts of these substances can vary. E.g., the amount of urea will increase during starvation and when a person is eating a high-protein diet and creatinine excretion will increase after physical activity. 

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