Enkephalin binding systems in human plasma. II: Leu-enkephalin serum albumin interaction.

Enkephalins are released into the bloodstream of mammals by the adrenal medulla. Once they are in the blood, these peptides undergo a fairly rapid hydrolysis by several plasma-contained enzymes. However, a fraction of the enkephalins present in the plasma are bound to the serum albumin, and the bound peptides are almost completely intact even after a long incubation in the presence of serum enzymes. Therefore, it seems possible that the interaction with serum albumin can maintain the functional integrity of the circulating enkephalins. Moreover, serum albumins are extremely well characterized proteins and, therefore, a suitable model for the study of protein-enkephalin interaction in general. The present work is a first step in the study of the mechanism of serum albumin-enkephalin interaction. Apparently, ionic parameters are important in the binding phenomenon. Furthermore, the serum albumin conformational status seems to be relevant in the binding. Finally, the binding is followed by a limited rearrangement of the protein molecule.