The breakdown and replacement of protein in the body.
Protein turnover consists of two processes: synthesis and degradation. Synthesis is described in the preceding section. The proteins synthesized by the body have a finite existence. They are subject to a variety of insults and modifications. Some of these modifications have been listed, for example, a prohormone is converted to an active hormone; an enzyme is activated or inactivated with the addition or removal of a substituent and so forth. Thus, it is that a dynamic state within the body exists with respect to its full complement of peptides and proteins. Some proteins have very short lifetimes and very rapid turnover times; other proteins are quite stable and long lived. Their turnover time is quite long. The estimate of the life of a protein, that is, how long it will exist in the body, is its half- life. A half-life is the time interval that occurs when half of the amount of a compound synthesized at time X will have been degraded.