Thiol dependent isomerization of bovine albumin.

Albumin isomerizes to the aged form in the presence of cysteine at pH 8.9 and low ionic strength. Albumins with a high fatty acid and Cu(II) content do not produce isomers, and recover this capacity after an acid expansion. Isomers have the free thiol group fully oxidized (non-mercaptalbumin) and have been isolated for the first time from aged albumins by anion and cation exchange chromatography. Isomers have a higher susceptibility to limited tryptic digestion and show a decrease in the fluorescence of bound dansylamide, a typical marker of site I. Aging in the presence of phenylarsine oxide, which complexes vicinal thiols, impairs the formation of isomers and increases the free -SH groups of albumin.