Corticotrophin‐like Intermediary Lobe Peptide as a Marker of Alternate Pro‐opiomelanocortin Processing in Acth‐producing Non‐pituitary Tumours

In order to evaluate which of human (h) corticotrophin-like intermediary lobe peptide (CLIP) or h beta-melanocyte stimulating hormone5-22 (h beta MSH5-22) was the better marker of alternate pro-opiomelanocortin (POMC) processing, both peptides were simultaneously sought in the same tissue extracts from a normal human pituitary, six corticotrophic adenomas, and four non-pituitary tumours responsible for an ectopic ACTH syndrome. Human CLIP was detected using a combination of gel exclusion chromatography and two different radioimmunoassays (RIAs): a mid-ACTH RIA which recognized ACTH but not CLIP, and a COOH-ACTH RIA which recognized both molecules. Human beta MSH5-22 had been measured previously. Neither hCLIP nor h beta MSH5-22 were detected in the normal or tumoural pituitaries. The four non-pituitary tumours, in contrast, contained both peptides; the hCLIP and h beta MSH5-22 ratios (CLIP/CLIP + ACTH and h beta MSH5-22/h beta MSH5-22 + h gamma LPH) ranged from 40 to 94% and from 24 to 46%, respectively. In a given tissue the hCLIP ratio was always higher than the h beta MSH5-22 ratio. hCLIP is therefore the better marker of alternate POMC processing.