ABSTRACT
In micro-organisms, animals and plants, the proteins are subject to a continuous turnover, and the intracellular concentration of a given protein depends on rates of synthesis and degradation. Most proteins have a lifespan shorter than that of the cell; in young leaves of Lemna minor, the average half-life of proteins was estimated as 7 days (Davies, 1982). Such observations indicate that the metabolic flux of nitrogen and amino acids is both cyclic and permanent. Although the turnover of proteins seems to be an energyconsuming and unprofitable process at first sight, studies on a large number of organisms have shown that proteolysis ensures many physiological and developmental functions of the cell and its constituents.
