Human γ-Lipotropin Radioimmunoassay: Identification of Immunoreactive γ-Lipotropin in Human Plasma and Tissue*

Abstract

Disagreement exists concerning the relative contributionsto total plasma immunoreactive human lipotropin(hLPH) made by hβLPH and its amino-terminal fragment,hγLPH [hβLPH-(l-58)]. Using an antiserum (R1547) whichrequires the free COOH-terminal Asp⁵⁸ residue of hγLPH for fullaffinity and reacts only 1% as well with hβLPH and antisera (R3and G106) that react with both LPHs, we examined gel chromatographyeluate fractions of plasma extracts from one normalsubject under basal conditions and another after metyraponeadministration, of plasma or plasma extracts from patients withACTH/LPH hypersecretion of various causes, of an extract twonormal pituitary glands, and an extract of an ectopic ACTH/LPH-secreting tumor. Immunoreactive hγLPH was always amajor, frequently the predominant, and sometimes the onlyimmunoreactive LPH observed. We also observed in plasma ortissue of two patients with ectopic ACTH/LPH syndrome apeptide whose immunoreactivity and apparent molecular sizewere consistent with those of octadecapeptide human /?MSH, amolecule that is not thought to exist in normal man. Thesestudies demonstrate that hγLPH is a major LPH component inplasma and tissues and indicate that the hγLPH RIA providesa reliable estimate of the hγLPH concentration in plasma withoutprior chromatography.