Lactoperoxidase catalytically oxidize hydrogen sulfide via intermediate formation of sulfheme derivatives

The biological chemistry of hydrogen sulfide (H₂S) with physiologically important heme proteins is in the focus of redox biology research. In this study, we investigated the interactions of lactoperoxidase (LPO) with H₂S in the presence and absence of molecular dioxygen (O₂) or hydrogen peroxide (H₂O₂). Under anaerobic conditions, native LPO forms no heme-H₂S complex upon sulfide exposure. However, under aerobic conditions or in the presence of H₂O₂ the formation of both ferrous and ferric sulfheme (sulfLPO) derivatives was observed based on the appearances of their characteristic optical absorptions at 638 nm and 727 nm, respectively. Interestingly, we demonstrate that LPO can catalytically oxidize H₂S by H₂O₂ via intermediate formation of relatively short-lived ferrous and ferric sulfLPO derivatives. Pilot product analyses suggested that the turnover process generates oxidized sulfide species, which include sulfate (SO₄²⁻) and inorganic polysulfides (HS_x⁻; x = 2–5). These results indicated that H₂S can serve as a non-classical LPO substrate by inducing a reversible sulfheme-like modification of the heme porphyrin ring during turnover. Furthermore, electron paramagnetic resonance data suggest that H₂S can act as a scavenger of H₂O₂ in the presence of LPO without detectable formation of any carbon-centered protein radical species, suggesting that H₂S might be capable of protecting the enzyme from radical-mediated damage. We propose possible mechanisms, which explain our results as well as contrasting observations with other heme proteins, where either no sulfheme formation was observed or the generation of sulfheme derivatives provided a dead end for enzyme functions.