Limited pepsin digestion of bovine plasma albumin

Abstract

Limited pepsin digestion of bovine plasma albumin at pH 3.7 and 25 °C in the presence of octanoic acid gave two fragments, A and B, each in about 16% yield. In the absence of octanoic acid fragment A was rapidly degraded further into smaller fragments. Sodium dodecylsulfate gel electrophoreses and amino acid analyses of fragments A and B indicated their molecular weights to be about 29,000 and 34,000, respectively. Comparative studies of the cyanogen bromide peptides of fragments A and B with those of intact albumin established that fragments A and B represent, respectively, the carboxyl and the amino terminal portions of the albumin molecule. In Tris-HCl buffer (pH 7.95) at 25 °C, fragment A has one primary binding site for octanoic acid with a binding constant about one-eighth of that of albumin. This binding constant is doubled in the presence of an equimolar amount of fragment B, although fragment B itself shows very weak activity, less than one three-hundreth of that of albumin. l- and d-tryptophans competitively bind at the same primary octanoate binding site of fragment A, just as is the case with albumin. These findings together with those of other studies suggest that the albumin molecule might consist of several compact regions and that the interactions of these regions within the molecule vary with the solvent environment and upon binding of organic ligands.