PROOPIOMELANOCORTIN-DERIVED PEPTIDES

ACTH is generated in the pituitary gland by the proteolytic processing of a polypeptide precursor proopiomelanocortin (POMC). Other POMC-derived peptides, such as the N-terminal fragment, the joining peptide, beta-lipotropin, gamma-lipotropin, and beta-endorphin, are consecrated with ACTH by exocytosis on an equimolar basis. All of these peptides have been measured in blood by radioimmunoassay or immunoradiometricassay, and their levels are markers of the overall coricotroph function. The normal ratio between the various circulating POMC peptides is altered in chronic renal failure in which lipotropins and joining peptide are high, whereas ACTH is normal. In rare pituitary macroadenomas, and more often in nonpituitary tumors responsible for the ectopic ACTH syndrome, POMC processing is altered qualitatively. Intact POMC or abnormal fragments such as CLIP or h beta MSH(5-22) are produced and secreted; their detections by specific radioimmunoassay of immunoradiometricassay may help the diagnostic investigation and the therapeutic follow-up examination.