Human Gamma-lipotropin Radioimmunoassay: Identification of Immunoreactive Gamma-lipotropin in Human Plasma and Tissue

Disagreement exists concerning the relative contributions to total plasma immunoreactive human lipotropin (hLPH) made by h beta LPH and its amino-terminal fragment, h gamma LPH [h beta LPH-(1-58)]. Using an antiserum (R1547) which requires the free COOH-terminal Asp58 residue of h gamma LPH for full affinity and reacts only 1% as well with h beta LPH and antisera (R3 and G106) that react with both LPHs, we examined gel chromatography eluate fractions of plasma extracts from one normal subject under basal conditions and another after metyrapone administration, of plasma or plasma extracts from patients with ACTH/LPH hypersecretion of various causes, of an extract two normal pituitary glands, and an extract of an ectopic ACTH/LPH-secreting tumor. Immunoreactive h gamma LPH was always a major, frequently the predominant, and sometimes the only immunoreactive LPH observed. We also observed in plasma or tissue of two patients with ectopic ACTH/LPH syndrome a peptide whose immunoreactivity and apparent molecular size were consistent with those of octadecapeptide human beta MSH, a molecule that is not thought to exist in normal man. These studies demonstrate that h gamma LPH is a major LPH component in plasma and tissues and indicate that the h gamma LPH RIA provides a reliable estimate of the h gamma LPH concentration in plasma without prior chromatography.