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FIGURE 5-1 Major Pathways of Protein Degradation. Source: Young, V.R. and Y.M. Yu, 1996. Protein and amino acid metabolism. Pp. 159-200 in Nutrition and Metabolism in the Surgical Patient, 2nd Edition, Josef E. Fischer, ed. Boston: Little, Brown and Company.

mitochondrial proteins are degraded through energy-dependent pathways requiring hydrolysis of ATP (Gottesman et al., 1997). Extracellular and membrane proteins are degraded through the lysosomal pathway. Although the latter route is not an energy-dependent process in itself, energy is required for proton movement into and maintenance of the acid environment within the lysosome. In Table 5-5, Mitch and Goldberg (1996) have summarized some of the conditions that alter muscle protein degradation through the energy-dependent ubiquitin-proteosome pathway. It is clear from this table that the

TABLE 5-5 Conditions That Alter Muscle Protein Degradation Through the Ubiquitin-Proteosome Pathway

Rat Models

Humans

Increased Protein Degradation

 

Fasting

Eating disorders

Metabolic acidosis

Renal tubular defects

Renal failure

Uremia

Diabetes mellitus

Diabetes mellitus

Thermal injury

Bums

Endotoxin, bacteria

Sepsis

Tumors

Cancer cachexia

Glucocorticoids

Cushing's Syndrome

Decreased Protein Degradation

 

Deficient dietary protein

Malnutrition

Hypothyroidism

Hypothyroidism

 

SOURCE: Adapted from Mitch and Goldberg, 1996



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