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Effect of Cyclic Adenosine 3',5'-Monophosphate on the Incorporation of 3H-leucine Into Polypeptides by Beef Thyroid Polysomes in Vitro

Departments of Obstetrics and Gynecology, Physiology, Medicine and Biochemistry, Vanderbilt University School of Medicine, Nashville, Tennessee 37203

Abstract

Polysomes were isolated from beef thyroids and then analyzed in a cell-free system in order to determine whether adenosine 3',5'- cyclic monophosphate (cAMP) was capable of eliciting a direct effect on protein synthesis. When concentrations of ATP and GTP were present which resulted in optimal incorporation of 3H-leucine into protein in vitro (i.e., 1 and 1.6 mM, respectively), addition of cAMP to the cellfree system did not cause an increase in protein synthesis. On the other hand, when suboptimal amounts of GTP were used in the system, addition of cAMP resulted in a slight stimulation of 3H-leucine incorporation. Similarly, if ATP was omitted from the incubation mixtures, cAMP caused up to a 2-fold increase in the rate of protein synthesis. Under optimal conditions for protein synthesis, a decrease in incorporation was observed by the addition of cAMP, dibutyryl cAMP or 5'-AMP. This fact, together with the removal of inhibition by cAMP in the presence of theophylline, suggests that degradation products of cAMP were inhibitory to protein synthesis. These results indicate that cAMP does not directly stimulate pi-otein synthesis at the translational level. Instead, cAMP may substitute for GTP and/or ATP at certain steps in protein synthesis when optimal levels of these nucleotides are not present. (Endocrinology 89: 70, 1971)

Footnotes

¹ Present address: Department of Surgery, University of Oregon Medical School, Portland, Oregon 97201.

Received October 23, 1970.