Control of protein degradation in reticulocytes and reticulocyte extracts by hemin.

Previous studies have established that hemin stimulates production of globin and other proteins in reticulocytes and nonerythroid cells. Such studies generally equated increased production with more rapid synthesis and ignored the additional possibility that hemin decreases protein degradation. Addition of hemin to cell-free extracts of rabbit reticulocytes consistently inhibited the rapid ATP-dependent degradation of abnormal globins containing puromycin or an amino acid analog, of apohemoglobin, and of apomyglobin. In addition hemin decreased the slower degradation of normal hemoglobin. Hemin reduced to a similar extent the degradation of nonheme proteins, including lysozyme, a-casein, and mixed Escherichia coli proteins. Therefore, the primary action of hemin probably involves an inhibition of the ATP-dependent proteolytic system rather than a specific stabilization of globin resulting from heme binding. Accordingly, the degradation of globin was much more sensitive to exogenous hemin in the presence of ATP than in its absence. Addition of hemin to intact reticulocytes also inhibited the degradation of abnormal and normal hemoglobin while stimulating protein synthesis. These actions of hemin on synthesis and degradation appear additive in promoting net protein accumulation and thus preventing an imbalance between the production of globin and heme. However, the relative importance of the effects of hemin on synthesis and proteolysis varied in different cell preparations. Thus, in certain preparations, the enhanced accumulation of globin induced by hemin resulted primarily from decreased degradation, while in others enhanced synthesis appeared to be the more important effect. The basis for this variability in the responses of whole cells to hemin is unclear.