High concentrations of LPO are present in human airway secretions, suggesting a function of LPO in the airway host defense against respiratory diseases ( 28). Its main physiological function is the oxidation of thiocyanate (SCN −) to the bactericidal hypothiocyanite ( −OSCN), which makes LPO an important part of the innate host defense ( 25, – 27). LPO is a heme-containing mammalian enzyme found in exocrine secretion liquids, including saliva, tears, milk, and the lining fluids of the airway. Whether urate is an important physiological substrate for lactoperoxidase (LPO) is unknown. It has also been suggested to be a substrate for the peroxidase cycle of prostaglandin synthase ( 24). Recently, kinetic studies by us revealed urate to be a physiological substrate for the neutrophil enzyme myeloperoxidase (MPO) 3 ( 23). Indeed, it was shown early that mammalian peroxidases oxidize urate to allantoin ( 21, 22). This reduction potential also makes urate a likely substrate for heme peroxidases.
The major exception is activation of the NLRP3 inflammasome by uric acid crystals in gout ( 15, – 17). High concentrations of plasma uric acid are associated with adverse outcomes in numerous inflammatory diseases ( 9, – 14), yet the mechanisms linking it to a particular pathology for the most part remain obscure. In contrast to other mammals, in humans urate is considered the end product of purine metabolism because the urate-catabolizing enzyme uricase has been lost during primate evolution ( 7, 8). It accumulates in its mono-anionic form urate to produce high concentrations in cells and extracellular fluids such as lymphatic, interstitial, synovial, cerebrospinal, and respiratory tract lining fluids ( 1), but its precise physiological function remains unknown, although it is suggested to be an antioxidant ( 2, – 5) and a danger signal for inflammatory tissue damage ( 6). Uric acid is an enigma in human metabolism.
Our findings establish urate as a likely physiological substrate for LPO that will influence host defense and give rise to reactive electrophilic metabolites. When hydrogen peroxide was added to saliva, oxidation of urate was dependent on its concentration and peroxidase activity. Allantoin was present in human saliva and associated with the concentration of LPO. Similarly, hypothiocyanite-dependent killing of Pseudomonas aeruginosa was inhibited by urate. At physiologically relevant concentrations, urate competed effectively with thiocyanate, the main substrate of LPO for oxidation, and inhibited production of hypothiocyanite. During urate oxidation, LPO was diverted from its peroxidase cycle because hydrogen peroxide reacted with compound II to give compound III. Using stopped-flow spectroscopy, we determined rate constants for the reaction of urate with compound I ( k 1 = 1.1 × 10 7 m −1 s −1) and compound II ( k 2 = 8.5 × 10 3 m −1 s −1).
In the presence of superoxide, high yields of hydroperoxides were formed by LPO and urate. Urate was oxidized by LPO to produce the electrophilic intermediates dehydrourate and 5-hydroxyisourate, which decayed to allantoin. We now demonstrate that urate is a good substrate for bovine LPO. LPO is a mammalian peroxidase that plays a key role in the innate immune defense by oxidizing thiocyanate to the bactericidal and fungicidal agent hypothiocyanite. Whether it reacts sufficiently rapidly with lactoperoxidase (LPO) to act as a physiological substrate remains unknown. It may act as a danger signal, an antioxidant, or a substrate for heme peroxidases. The physiological function of urate is poorly understood.
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