The adrenocorticotropin differs less in its structure than do the growth hormone. The ACTH of beef and human contains 39 and 41 amino-acids respectively and its structure has been fully worked out by C.H.Li, and his collaborators at Berkeley, California and by Bell and his associates.
Human ACTH differs in having two additional amino-acids (valine and tyrosine) near the amino end of the chain.
The structure of ACTH is of interest in that it contains a sequence in common with the smaller molecules of the melanocyte stimulating hormones, namely, methionine, glutamine, histidine, phenylalanine, arginine, tryptophan, glycine.
In consequence, ACTH has MSH (melanin stimulating hormone) activity as well. The MSH hormone has much smaller molecule than ACTH.
The MSH end of the ACTH molecule is apparently not its functional end as regards stimulation of the adrenal cortex is concerned.
This stimulation is fully reproduced by a synthetic polypeptide comprising the last 23 amino- acids of the ACTH chain (Hoffman).
The adrenal cortex is the main target organ upon which ACTH acts, although a number of extra-adrenal functions have also been reported. The steroid hormonal secretions of adrenal cortex act throughout the body to
(i) Increase the rate of gluconeogenesis;
(ii) Increase the rate of protein catabolism so that blood glucose concentration may greatly be increased
(iii) Increase the rate of fat-catabolism. Therefore, indirectly corticotropin has profound effects on carbohydrate, protein and fat metabolism.
ACTH is used as a non-disease specific. It is used in the treatment of hypersensitivities and inflammatory reactions and also in the treatment of rheumatoid arthritis and acute rheumatic fever.
It is also used in the treatment of chronic and intractable cases of asthma. Its deficiency also causes acute psoriasis and dermatitis.
Its over secretion causes cushing’s disease which is characterized by obesity of the trunk (especially of the abdomen), face, and buttocks but not the limbs; cyanosis of the face, hands, and feet, pigmentation of the skin, and excessive growth of hair ; women may grow a mustache or beard ; demineralization of the bones ; loss of sexual functions and hyperglycemia and glucosuria.
It is probable that its production is initiated and increased by the corticotropin releasing factor secreted by the neurosecretory cells of the hypothalamus; this releasing factor is transmitted down into the adenohypophysis through the portal system and causes the production of corticotropin hormone.
Its production is also increased by adrenaline in the blood and decreased by circulating cortical steroids, so that there is some automatic control of the latter.