Covalent binding of 4-nitrobenzyl mercaptan S-sulfate to the sulfhydryl groups of hepatic cytosolic proteins and bovine serum albumin with mixed disulfide bond formation

Abstract

4-Nitrobenzyl [ 35S]mercaptan S-sulfonic acid ([ 35S]NBM S-sulfate), a new type of reactive metabolite of the thiol [ 35S]NBM in rat liver cytosol fortified with 3′-phosphoadenosine 5′-phosphosulfate, bound rapidly and covalently at pH 7.4 and 37°C to the sulfhydryl groups of rat liver cytosolic proteins with formation of disulfide bonds. From the radioactive proteins was isolated and identified the sole amino acid adduct, S-([ 35S]NBM)cysteine, after their acid hydrolysis under the anaerobic conditions. Bovine serum albumin (BSA), a model protein with a single SH group, also reacted readily with radioactive NBM S-sulfate to form a disulfide bond in stoichiometric manner. S-([ 35S]NBM)cysteine was also isolated and identified as the sole amino acid adduct from the well-washed, radioactive BSA after the same anaerobic acid hydrolysis. A normal hepatic level of GSH not only retarded the BSA-NBM adduct formation completely, but also detached the radioactivity from BSA by the reduction of the disulfide bond with formations of [ 35S]NBM and its disulfide. Of twenty-one amino acids examined at pH 7.4 and 37°C, only cysteine reacted with NBM S-sulfate and afforded S-(NBM)cysteine with concomitant formations of S-sulfocysteine, cystine, NBM, and its disulfide.