Abstract

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.