Studies on Xanthine Oxidase: The Reduction of Nitrates

"Repurposing" of Xanthine Oxidoreductase as a Nitrite Reductase: A New Paradigm for Therapeutic Targeting in Hypertension

SIGNIFICANCE

In contrast to nitric oxide (NO), which has well-established, important effects in regulation of cardiovascular homeostasis, its oxidative metabolite nitrite has, until recently, been considered to be of minor functional significance.

RECENT ADVANCES

However, this view of nitrite has been radically revised over the past 10 years with evidence now supporting a critical role for this anion as a storage form of NO.

CRITICAL ISSUES

Importantly, while hypoxia and acidosis have been shown to play a pivotal role in the generation of nitrite to NO, a number of mammalian nitrite reductases have been identified that facilitate the reduction of nitrite. Critically, these nitrite reductases have been demonstrated to operate under physiological pH conditions and in normoxia, extending the functional remit of this anion from an ischemic mediator to an important regulator of physiology. One particular nitrite reductase that has been shown to operate under a wide range of environmental conditions is the enzyme xanthine oxidoreductase (XOR).

FUTURE DIRECTIONS

In this review, we discuss the evidence supporting a role for XOR as a nitrite reductase while focusing particularly on its function in hypertension. In addition, we discuss the potential merit in exploiting this activity of XOR in the therapeutics of hypertension.

Nitrite and nitrite reductases: from molecular mechanisms to significance in human health and disease

Nitrite, previously considered physiologically irrelevant and a simple end product of endogenous nitric oxide (NO) metabolism, is now envisaged as a reservoir of NO to be activated in response to oxygen (O(2)) depletion. In the first part of this review, we summarize and compare the mechanisms of nitrite-dependent production of NO in selected bacteria and in eukaryotes. Bacterial nitrite reductases, which are copper or heme-containing enzymes, play an important role in the adaptation of pathogens to O(2) limitation and enable microrganisms to survive in the human body. In mammals, reduction of nitrite to NO under hypoxic conditions is carried out in tissues and blood by an array of metalloproteins, including heme-containing proteins and molybdenum enzymes. In humans, tissues play a more important role in nitrite reduction, not only because most tissues produce more NO than blood, but also because deoxyhemoglobin efficiently scavenges NO in blood. In the second part of the review, we outline the significance of nitrite in human health and disease and describe the recent advances and pitfalls of nitrite-based therapy, with special attention to its application in cardiovascular disorders, inflammation, and anti-bacterial defence. It can be concluded that nitrite (as well as nitrate-rich diet for long-term applications) may hold promise as therapeutic agent in vascular dysfunction and ischemic injury, as well as an effective compound able to promote angiogenesis.

Reduction of nitrite to nitric oxide catalyzed by xanthine oxidoreductase

Xanthine oxidase (XO) was shown to catalyze the reduction of nitrite to nitric oxide (NO), under anaerobic conditions, in the presence of either NADH or xanthine as reducing substrate. NO production was directly demonstrated by ozone chemiluminescence and showed stoichiometry of approximately 2:1 versus NADH depletion. With xanthine as reducing substrate, the kinetics of NO production were complicated by enzyme inactivation, resulting from NO-induced conversion of XO to its relatively inactive desulfo-form. Steady-state kinetic parameters were determined spectrophotometrically for urate production and NADH oxidation catalyzed by XO and xanthine dehydrogenase in the presence of nitrite under anaerobic conditions. pH optima for anaerobic NO production catalyzed by XO in the presence of nitrite were 7.0 for NADH and </=6.0 for xanthine. Involvement of the molybdenum site of XO in nitrite reduction was shown by the fact that alloxanthine inhibits xanthine oxidation competitively with nitrite. Strong preference for Mo=S over Mo=O was shown by the relatively very low NADH-nitrite reductase activity shown by desulfo-enzyme. The FAD site of XO was shown not to influence nitrite reduction in the presence of xanthine, although it was clearly involved when NADH was the reducing substrate. Apparent production of NO decreased with increasing oxygen tensions, consistent with reaction of NO with XO-generated superoxide. It is proposed that XO-derived NO fulfills a bactericidal role in the digestive tract.

THE EFFECTS OF SALTS ON THE NITRITASE AND LACTIC ACID DEHYDROGENASE ACTIVITY OFMICROCOCCUS HALODENITRIFICANS

With Micrococcus halodenitrificans, maximum production of nitritase occurs just after nitrite has been reduced completely to nitrogen. Maximum denitrifying activity takes place at pH 7.6 in the presence of 0.38 to 0.75 molar sodium chloride or sodium bromide. The activity is slightly less in the presence of lithium chloride but reaches a peak at 0.19 to 0.38 molar concentration. The reduction of nitrate to nitrite is inhibited by azide whereas reduction of nitrite to nitrogen is not, suggesting the possibility that at least two separate enzymes may be involved in these reactions. The lactic acid dehydrogenase of this organism shows maximum activity in the presence of 0.75 molar sodium chloride.