Intracellular breakdown

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. 

Poole, B., Ohkuma, S., and Warburton, M. (1978). Some aspects of the intracellular breakdown of exogenous and endogenous proteins. In Segal H.L. and Doyle D.J., eds. Protein Turnover and Lysosome Function. Academic Press, New York, NY USA. pp. 43-58. 

No literature is available on the subject. It should be emphasized that the present knowledge of the enzymatic mechanisms for the intracellular breakdown of proteins in general is meager (Fruton, 1963). 

The many enzymes capable of splitting peptide bonds and of degrading protein to various extents are, of com, among the oldest and best known of all enzymes yet, the intracellular breakdown of proteins is still at least as obscure as their synthesis. 

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