Enkephalin-binding protein in human blood.

An enkephalin-binding protein was found in human plasma and serum. The protein was partially purified by DEAE-cellulose column chromatography. The binding of [3H]leucine-enkephalin to this protein was competitively inhibited by unlabeled leucine- and methionine-enkephalin and various peptide hormones such as beta-endorphin and glucagon, but not by Leu-enkephalin-amide. The fact that amide derivatives of leucine-enkephalin and methionine-enkephalin did not inhibit the binding suggests that c-terminuses of enkephalins might have an important part in binding the protein. From these results, physiological roles of the enkephalin-binding protein are discussed.