Oxidation increases the proteolytic susceptibility of a localized region in rhodanese.

Abstract

For the first time, the enzyme rhodanese has been proteolytically cleaved to give species that most likely correspond to individual domains. This indicates cleavage can occur in the interdomain tether. Further, the conditions for cleavage show that availability of the susceptible bond(s) depends on conformational changes triggered by oxidative inactivation. Rhodanese, without persulfide sulfur (E), was oxidized consequent to incubation with phenylglyoxal, NADH, or hydrogen peroxide. The oxidized enzyme (Eox) was probed using the proteolytic enzymes endoproteinase glutamate C (V8), trypsin, chymotrypsin, or subtilisin. The proteolytic susceptibility of Eox, formed using hydrogen peroxide, was compared with that of E and the form of the enzyme containing transferred sulfur, ES. ES was totally refractory to proteolysis, while E was only clipped to a small extent by trypsin or V8 and not at all by chymotrypsin or subtilisin. Eox was susceptible to proteolysis by all the proteases used, and, although there were some differences among the proteolytic patterns, there was always a band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis corresponding to Mr = 16,500. This was the only band observed in addition to the parent species (Mr = 33,000) when Eox was digested with chymotrypsin, and conservation of total protein was observed after digestion up to 90 min. No additional species were observable on silver staining, although there was some indication that the band at 16,500 might be a doublet. The results are consistent with the occurrence of a conformational change after oxidation that results in increased exposure and/or flexibility of the interdomain tether which contains residues that meet the specificity requirements of the proteases used.