Mucosal protection against sulphide: importance of the enzyme rhodanese

Background: Hydrogen sulphide (H2S) is a potent toxin normally present in the colonic lumen which may play a role in ulcerative colitis (UC). Two enzymes, thiol methyltransferase (TMT) and rhodanese (RHOD), are thought to be responsible for sulphide removal but supportive evidence is lacking. Aims: To determine the distribution of TMT and RHOD in different sites throughout the gastrointestinal tract and their efficacy as detoxifiers of H2S. Methods: Enzyme activities were measured in normal tissue resected from patients with cancer. TMT and RHOD activities were determined using their conventional substrates, 2-mercaptoethanol and sodium thiosulphate, respectively. For measurement of H2S metabolism, sodium sulphide was used in the absence of dithiothreitol. Thiopurine methyltransferase (TPMT), which in common with TMT methylates sulphydryl groups but is not thought to act on H2S, was also examined. Results: TMT, RHOD, and TPMT activities using their conventional substrates were found throughout the gastrointestinal tract with highest activity in the colonic mucosa. When H2S was given as substrate, no reaction product was found with TMT or TPMT but RHOD was extremely active (Km 8.8 mM, Vmax 14.6 nmol/mg/min). Incubation of colonic homogenates with a specific RHOD antibody prevented the metabolism of H2S, indicating that RHOD is responsible for detoxifying H2S. A purified preparation of RHOD also detoxified H2S. Conclusions: RHOD, located in the submucosa and crypts of the colon, is the principal enzyme involved in H2S detoxication. TMT does not participate in the detoxication of H2S.