Heat inhibition of reticulocyte protein synthesis Evidence for a mechanism independent of the hemin-controlled repressor

Abstract

Incubation of rabbit reticulocytes at 45°C results in a prompt but reversible decrease in protein synthesis and a concomitant conversion of polyribosomes to smaller aggregates. These effects occur even in the presence of 100 μM hemin in the incubation medium. There is also inhibition of heme synthesis, but this occurs at a later time than the effect on protein synthesis. The inhibition of heme synthesis results from a decrease in activity of δ-aminolevulinic acid synthetase. This decrease of heme synthesis appears to be secondary to the inhibition of protein synthesis with resultant accumulation of intramitochondrial heme (which will decrease δ-aminolevulinic acid synthetase activity). An inhibitor of reticulocyte cell-free protein synthesis formed in the post-ribosomal supernatants of cells incubated at both 45 and 37°C, but not at 0°C. No temporal or quantitative differences in the amount of this inhibitor from cells treated at either 37 or 45°C was apparent. The inhibitor was not found in the fraction where the hemin-controlled repressor is isolated. It is concluded that heat inactivation of intact reticulocyte protein synthesis does not depend upon a decrease in heme synthesis, heme concentration or generation of the hemin-controlled repressor. Furthermore, it appears that the inhibitor formed in the post-ribosomal supernatant cannot be the sole cause of the heat inhibition of protein synthesis.