Use of chronically labeled animals in the study of a "metabolically inert" protein.

Abstract

The use of acute labeling techniques (pulse labeling) has been successful in tracing and locating rapid metabolic pathways in vivo. This procedure was used in the classical experiments of Schoenheimer [1] on labile cellular proteins which gave rise to the concept of the dynamic state of body constituents. The concept described protein turnover as a continual process of synthesis and degradation as evidenced by the active uptake and release of isotopic compounds (see Fig. 1). Turnover represented the occurrence of synthesis which cannot be detected analytically because it was exactly balanced by breakdown [2]. Acute and chronic isotopic studies have demonstrated that the fibrous proteins of muscle and connective tissues have the least active turnover and are considered to be relatively inert metabolically [3, 4]—inert in the sense that the proteins were nonrenewable. However, these studies could not distinguish between essentially non isotopic turnover (nonsynthetic and nondestructive) of macromolecules and absolute inertness.