Control of Haemoglobin Synthesis

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Abstract
A detailed examination of the kinetics of protein synthesis in rabbit reticulocytes in the presence of the iron chelating agent 2,2'-dipyridyl showed that between 30 degrees C and 42 degrees C there were characteristically two distinct phases of protein synthesis. An initial phase (I), in which no inhibition of protein synthesis was apparent, was followed by a gradual decline in the rate of protein synthesis leading to the second phase (II) in which protein synthesis occurred at a linear but inhibited rate for extended periods. In contrast, below 30 degrees C, incubation in the presence of dipyridyl caused no inhibition of protein synthesis. Between 30 degrees C and 42 degrees C the duration and amount of protein synthesis occurring in phase I before the onset of inhibition were inversely related of the inhibition as was the final rate of incorporation in phase II. During phase II, a partial reversal of the inhibition caused by dipyridyl was obtained by lowering the incubation temperature. This resulted in a burst of protein synthesis at the uninhibited rate until the amount of protein synthesis reached the same level as that in reticulocytes maintained continuously with dipyridyl at the lower incubation temperature. This burst of synthesis was observed in reticulocytes which had been held in phase II for as long as 90 min. It was also possible to reverse the inhibition by addition of haemin to cells in phase II. At any particular incubation temperature, a fixed number of rounds of protein synthesis had to occur before the onset of phase II became apparent. By the use of puromycin we showed that this was not a requirement for the synthesis of globin or of any other protein. We believe that this critical amount of protein synthesis reflects the residual ability of reticulocytes to initiate new protein chains in the absence of concurrent haem synthesis. Reticulocytes preincubated in the presence of cobaltous ions showed almost no inhibition of protein synthesis upon subsequent incubation with dipyridyl. The results are compared to those obtained in reticulocyte lysates and are discussed in terms of current theories to account for control of protein chain initiation by haemin.