Characterization of selenium-binding protein 1 and its orthologues in Caenorhabditis elegans

Selenium-binding protein 1 (SELENBP1) was recently identified as human methanethiol oxidase (MTO). Through its enzymatic function, it catabolizes methanethiol, a highly toxic, repulsive-smelling gas into hydrogen sulfide (H2S), hydrogen peroxide (H2O2) and formaldehyde. Methanethiol is predominantly produced by the microbiota, although endogenous biosynthetic pathways have been identified. Impaired metabolism, as it occurs in SELENBP1 loss-of-function mutants, is accompanied by exhalation of methanethiol and derivatives, manifesting extraoral halitosis. In context of this work, an assay was developed that allows for the measurement of MTO activity in cell lysates and recombinant proteins. Utilizing the assay, copper dependence and selenium independence of the MTO was demonstrated. To pursue this further, single amino acid changes were introduced into recombinant SELENBP1 using site-directed mutagenesis. In this regard several loss-of-function mutants were identified. Furthermore, ethanethiol, propanethiol, allyl mercaptan, methyl propanethiol, butanethiol and pentanethiol were identified as substrates of SELENBP1. Their metabolization provides new insights into the metabolism of microbiota- and plant-derived sulfur compounds. Expression of SELENBP1 and other H2S-related enzymes was demonstrated in the colon cancer cell line Caco-2. SEMO-1 was identified as a functional SELENBP1 ortholog in Caenorhabditis elegans. Knockout of SEMO-1 in C. elegans was accompanied by an inability to generate H2S from methanethiol. In addition, mean and maximum lifespan increased, resistance to oxidative stress was improved, yet nematodes were more sensitive to toxic concentrations of selenite. This work provides a comprehensive characterization of SELENBP1 in different model systems.