Enzyme linked immunosorbent assay for beta-endorphin in human plasma.

We established a highly sensitive and specific double-antibody enzyme linked immunosorbent assay for beta-endorphin (beta-EP). For competitive reactions, the beta-EP-antibody was incubated with beta-EP standard (or sample) and beta-D-galactosidase-labeled beta-EP (delayed addition). Free and antibody-bound labeled antigen were separated by using an anti-rabbit immunoglobulin G coated immunoplate. The enzyme activity on the plate was fluorometrically determined. The minimal detection limit was approximately 0.4fmol/well (10 pmol/l). Using this assay system, beta-EP-like immunoreactivity (-LI) in human plasma was determined. The level of beta-EP-LI in extracted human plasma from 6 normal subjects was 2.44 +/- 0.68 pmol/l. High performance liquid chromatography analysis of the plasma of a normal subject revealed a single immunoreactive form which eluted with the same retention time as that of synthetic beta-EP.