Determination of plasma beta-endorphin and beta-lipotropin by cation-exchange liquid chromatography and radioimmunoassay

In this method for rapid separation of beta-endorphin (beta-EP) and beta-lipotropin (beta-LPH) in plasma, most of the plasma proteins in a 0.5- to 2.0-mL sample of plasma are precipitated with acetonitrile at pH 4.7. beta-EP and beta-LPH in the supernate are completely separated by liquid chromatography on a cation-exchange column, with gradient elution with volatile buffers, and are eluted in 1.5- to 2.0-mL volumes. After lyophilization, the redissolved beta-EP and beta-LPH are determined by radioimmunoassay. The respective mean concentrations of immunoreactive beta-EP and beta-LPH in plasma of nonpregnant women were 4.0 (SD 2.0) and 5.1 (SD 3.0) pmol/L; during labor, 33.3 (SD 25) and 69.6 (SD 41) pmol/L; and in venous plasma from umbilical cords after spontaneous labor, 38.6 (SD 18) and 38.7 (SD 19) pmol/L.