Measurement of beta-endorphin in human plasma.

beta-endorphin has been identified in human plasma by means of gel filtration and a sensitive radioimmunoassay for human beta-endorphin (beta h-endorphin). Mean baseline plasma beta h-endorphin concentration in 5 individuals was 21 +/- 7.3 (SD) pg/ml (6.2 +/- 2.2 (SD) fmole/ml). Following metyrapone stimulation mean plasma concentration increased to 55.4 +/- 10.1 (SD) pg/ml (16.3 +/- 3.1 (SD) fmole/ml). The molar ratio of human beta-lipotropin (beta h-LPH) to beta h-endorphin was 2.2 in baseline plasma and 2.4 after metyrapone stimulation.