Human Plasma β-Endorphin-Like Peptides: A Rapid, High Recovery Extraction Technique and Validation of Radioimmunoassay*

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Abstract
This is a report of the development, calibration, and validation of a series of techniques required to measure β-endorphin (β-END)-like immunoreactivity in human plasma, including sieve and affinity chromatography. The RIA, which uses the antibody Brenda, is very sensitive (IC50 = 5–15 fmol/tube at a final concentration of 1:40,000). The extraction process, which uses the Sep-Pak C18 cartridge (Waters Associates, Inc.), is simple and rapid and has a recovery rate of more than 90%. It extracts proopiomelanocortin, β-lipotropin, and β-END. Physiological validation was provided by the measurement of β-END-like immunoreactivity in a pool of plasma of normal humans (2.25 fmol/ml plasma), two pregnant women at term (9.5 and 10.75 fmol/ml), and one patient with Nelson's disease (2 pmol/ml plasma).(J Clin Endocrinol Metab56: 992, 1983)