Studies on the biosynthesis and release of ACTH

Abstract

An attempt was made to clarify the mechanism of biosynthesis and release of ACTH.According to the method of Wool et al, the isolated rat anterior pituitary glands were incubated with 14C-phenylalanine, and the redioactivities incorporated into corticotropin (ACTH) and pituitary protein were measured.The corticosterone in rat plasma was determined by the fluorometric method of De Moor et al.The following results were obtained.1) The rate of incorporation of 14C-phenylalanine and 14C-alanine into ACTH fraction in the pituitary glands increased after adrenalectomy, but the rate of incorporation of 14C-isoleucine into ACTH fraction was not affected by the operation.It is known that isoleucine is not a component of amino acid residues of ACTH.Thus, the above-mentioned result does give support for identification of the isolated radioactive material with ACTH, although this is not evidence that ACTH and the radioactivity are identical.2) Puromycin, an inhibitor of protein biosynthesis, inhibited in vitro both ACTH and protein biosynthesis in rat anterior pituitary glands to the same degree.On the other hand, it was found that protein biosynthesis was far more suppressed by actinomycin C, an inhibitor of DNA dependent RNA bio-synthesis, in vitro than ACTH biosynthesis, and the difference was statistically significant. ACTH biosynthesis in the anterior pituitaries of rats that had been adrenalectomized 1 week before, was inhibited as well as that of normal rat pituitaries by either puromycin or actinomycin C.These results indicate that the biosynthesis of ACTH seems to involve the participation of polysome, as found in protein formation, although RNA specific for ACTH biosynthesis seems to undergo a slower turnover than RNA for protein biosynthesis, and that the similarity seems to exist between the mechanisms of ACTH biosynthesis in anterior pituitaries of normal and adrenalectomized rats.3) The rate of incorporation of 14C-phenylalanine into ACTH fraction in the pituitary glands increased after adrenalectomy and decreased by the treatment with prednisolone. However, protein biosynthesis in the pituitary glands was not affected either by adrenalectomy or by the treatment with prednisolone. Furthermore the addition of prednisolone in vitro failed to affect the incorporation of 14C-phenylalanine into ACTH fraction in the pituitary glands.It is generally accepted that biosynthesis and release of ACTH are controled by the concentration of glucocorticoids in blood through the feed-back mechanism.The above-mentioned results provide evidence that the biosynthesis of ACTH was specifically controled through the feed-back mechanism, irrelevant to protein biosynthe-sis in the pituitary gland, and the target point of the feed-back action of glucocorticoids might not be in the pituitary gland but in the hypothalamus.4) The concentration of corticosterone in the blood of rats elevated significantly in 30 minutes after intravenous injections of synthetic lysine-vasopressin or the peptide factor (s) extracted from the hypothalamus of rats. However, an extract of rat cerebral cortices by the same procedures as hypothalamic extracts did not cause elevation of corticosterone in the blood of rats.The biosynthesis of ACTH in the pituitary glands was increased only by the treatment with the peptide factor (s) from the hypothalamus.Synthetic lysine-vasopressin and peptide fraction from cerebral cortex had no effect on the biosynthesis of ACTH.Thus, it is considered that there is the peptide factor (s) which is different from vasopressin, and which posesses ACTH biosynthesizing and releasing activities in the rat hypothalamus.Further studies are required to elucidate whether this factor (s) and corticotropin releasing factor (CRF) are identical or not.