Sulfhemoglobin formation in human erythrocytes by cystalysin, an L‐cysteine desulfhydrase from Treponema denticola

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Abstract
Cystalysin, isolated from the oral pathogen Treponema denticola, is an l‐cysteine desulfhydrase (producing pyruvate, ammonia and hydrogen sulfide from cysteine) that can modify hemoglobin and has hemolytic activity. Here, we show that enzymatic activity of recombinant cystalysin depends upon stochiometric pyridoxal phosphate. The enzyme was not functional as an l‐alanine transaminase, and had a strong preference for l‐cysteine over d‐cysteine. Cystalysin preferred small α‐ l‐amino acids as substrates or inhibitors and was far more active towards l‐cysteine than towards the other standard amino acids that undergo pyridoxal phosphate‐dependent β‐elimination reactions (serine, threonine, tryptophan and tyrosine). Cystalysin tolerated small modifications to the carboxylate of l‐cysteine (i.e., the methyl and ethyl esters of l‐cysteine were good substrates), but the smallest possible peptide with an N‐terminal cysteine, l‐cysteinylglycine, was a very poor substrate. These results, combined with the implicit requirement for a free amine for pyridoxal phosphate‐dependent reactions, imply that cystalysin cannot catabolize cysteine residues located within peptides. Cystalysin has Michaelis‐Menten kinetics towards l‐cysteine, and there was little or no inhibition by ammonia, H2S, pyruvate and acetate. Human erythrocytes incubated with H2S or with cystalysin and cysteine primarily accumulated sulfhemoglobin and methemoglobin, along with minor amounts of choleglobin and protein aggregates. Erythrocytes retained the ability to reduce methemoglobin in the presence of H2S. Cystalysin could not modify hemoglobin when β‐chloroalanine was the substrate, indicating an absolute requirement for H2S production. Cystalysin appears to be an unregulated l‐cysteine catabolizing enzyme, with the resulting H2S production being essential to the atypical hemolytic activity.