On the conformation around the sulfhydryl group in human serum albumin.

Abstract

Human mercaptalbumin was treated with glutathione or cysteine at pH 7.0 to yield nonmercaptalbumin monomer in which the sulfhydryl group was bound with glutathione or cysteine, respectively. The ultraviolet difference spectra and optical rotatory dispersion of human serum albumin monomer, human serum albumin dimer, and human nonmercaptalbumin monomer, i.e., human mercaptalbumin-glutathione or human mer-captalbumin-cysteine compound were studied and compared. It was proved that: A) One tryptophanyl residue was exposed on the surface of protein and two tyrosyl residues, one of which was exposed and the other half-buried, were located near the sulfhydryl group in native human serum albumin. B) These chromophores in the neighborhood of the sulfhydryl group were inverted in the interior of the molecule with a fairly large decrease in the specific optical rotation of human serum albumin at 233 mμ as the sulfhydryl group was bound with a thiol group (from −9,300° to −8,000° by combination with glutathione, to −7,400° with cysteine, and to −8,700° with human serum albumin monomer). It seems to be possible to conclude that: 1) The reactive sulfhydryl group in the native albumin is located at the border between a polar helical segment and a hydrophobic area on the surface of the protein. 2) The half-buried tyrosyl residue near the sulfhydryl group is accommodated in a hydrophobic area located in the neighborhood of a helical segment or between helical segments of the protein fold, and may be the tyrosyl residues in the albumin molecule which is located at the closest position to the tryptophanyl residue.