Radioimmunoassay of leucine-enkephalin-like substance in human and canine plasma.

Fasting human plasma immunoreactive leu-enkephalin (ir-leu-enkephalin) measured by radioimmunoassay averages 54+/-10 pg/ml. The method depends on acidification of plasma to protect against destruction of the peptide, adsorption to XAD-2 resin, extraction from the resin by aqueous methanol and concentration by evaporation. Plasma enkephalin in the dog increased from 13 to 56 pg/ml following insulin-induced hypoglycemia.