The decreased stimulatory activity of hemin for globin synthesis at high concentrations of potassium salts in a rabbit reticulocyte lysate system.

The effect of various concentrations of hemin on globin synthesis was studied at various concentrations of potassium salts using a rabbit reticulocyte lysate system. The optimal concentration of hemin for globin synthesis was 10-30 microM at 100 mM and 200 mM K+. When the concentration of potassium salts was changed, globin synthesis without added hemin was optimal at 200 mM K+ or at 160 mM K+ and 60 mM Cl- and the globin synthesis at higher concentrations of potassium salts was similar to that at the optimal concentration of hemin. Using rabbit reticulocyte lysate, the binding of [35S]Met-tRNAf to the 40S ribosomal subunits was compared with that at the optimal concentration of added hemin. It was found that the binding without added hemin was inhibited greater at 100 mM KOAc than at 200 mM KOAc. But, a high concentration of hemin inhibited the binding at both KOAc concentrations. The postribosomal supernatant of the lysate was preincubated at various concentrations of KOAc and the inhibitory activity of the supernatant was measured in the lysate globin synthesis system. The data suggested that so-called "hemin-controlled translational inhibitor" was slowly formed and less active at the high KOAc concentration. Therefore, it is also suggested that stimulation of globin synthesis and Met-tRNAf binding to 40S ribosomal subunits in hemin-deficient lysate are due to the slower formation of HCI and the lower activity of HCI at a high K+ concentration. On the basis of these observations, 1) the different requirements of hemin for globin synthesis between reticulocytes and their lysates and 2) a possible physiological meaning of the findings are discussed.