Introduction

Xanthine oxidase is the enzyme required to produce uric acid by the breakdown of purine nucleotides. The uric acid itself, as well as the reactive oxygen species released during the enzymatic reaction, can have detrimental effects on the body. Gout is a common condition that occurs due to elevated uric acid production via this mechanism. As a result, many drugs have been formed to combat gout, by inhibiting xanthine oxidase enzyme activity. Moreover, regulations of this enzyme have proven to be effective for anti-cancer purposes.[1]

Molecular Level

Xanthine oxidase is from the molybdenum family that contains flavin adenine dinucleotide, one molybdenum, and four iron-sulfur attachments with a net molecular mass of approximately 290kD. Xanthine oxidase along with xanthine dehydrogenase are zymogens that are cleaved by the enzyme trypsin to be activated.[2]

Function

Xanthine oxidase (XO) is an enzyme found in many species, including humans, and primarily functions in the catabolism of purine nucleic acids. Through a series of reactions, the purines, guanine monophosphate (GMP), and adenosine monophosphate (AMP) get converted into either hypoxanthine or xanthine. The breakdown reaction of hypoxanthine and xanthine is catalyzed by xanthine oxidase into the uric acid. Uric acid then gets converted by the enzyme uricase into allantoin, a water-soluble molecule excreted in the urine. This enzyme is vital to mammalian cells as cell turnover is a constant process throughout the body, and endogenous or ingested purines are continually being degraded and renewed. Another form of xanthine oxidase is xanthine dehydrogenase (XD). Both forms are inter-convertible and catalyze the same reactions. However, they do differ in the cofactors used; where XO uses oxygen as a substrate, XD requires NADH to carry out the same reaction.[3][4][5]

The reaction carried out by XO reduces oxygen into a superoxide anion which eventually progresses into hydrogen peroxide. Hydrogen peroxide is a reactive oxygen species (ROS), the excess of which is toxic to individual cells. It also has links to aging and a multitude of conditions such as diabetes and neurodegenerative disorders such as Alzheimer disease.[6]

The product of xanthine oxidase activity, uric acid, has also proven to be harmful in previous literature. The urate behaves as a pro-oxidant and induces the formation of other radicals that have a tendency to oxidize lipid membranes, which explains the correlation between hyperuricemia and obesity. Reactive oxygen species naturally tend to damage cellular structures, DNA, and proteins, and consequently, oxidative stress caused by uric acid also has links to hypertension, diabetes, kidney disease, and cardiovascular diseases. Cardiovascular health, being endemic in North America, is currently being studied extensively for the risk level associated with xanthine oxidase activity. The activity of xanthine oxidase can also be monitored in corneal epithelium. As such, the deduction is that free radical damage is related to an oxidate eye injury that may result from the overactivity of xanthine oxidase.[7]

Pathophysiology

The deficiency of xanthine oxidase leads to a condition called hereditary xanthinuria type 1, attributed to a mutation in the XDH gene that leads to decreased amounts of xanthine oxidase production. Although rare, this condition can have detrimental effects on the individual, whereby a decreased metabolism of xanthine leads to elevated systemic levels of xanthine. The xanthine precipitates in the renal tubules and forms the rare xanthine kidney stones. If left untreated, this obstructs the renal parenchyma and can result in life-threatening renal failure. Patients also complain of recurrent episodes of muscle aches and joint pain. Patients with hereditary xanthuria type 1 are advised to limit their intake of proteins such as those found in meat products; this prevents the buildup of excess xanthine from protein breakdown and prevents the disease from worsening into a more severe state. Patients with type 2 of the condition have a normal expression of the XDH gene, but the final protein product fails to incorporate a sulfur molecule into its core, which ultimately limits the functionality of the protein. The symptoms of type 1 and type 2 hereditary xanthinuria are the same.  Studies have also indicated that patients with higher levels of XDH gene expression have a direct link to the survival rates of certain cancers. For example, patients with lung adenocarcinoma and a higher expression of the XDH gene had lower survival rates. Although this area is not particularly well studied, further research in this field is warranted.[8]

Clinical Significance

Manipulation of xanthine oxidase within the body has a plethora of pharmacological indications, the most significant being that of treating the symptoms of gout. Gout is a painful condition in which there is excess uric acid metabolism. The excess uric acid then precipitates and forms monosodium urate crystals within the joints and the kidney, causing painful gouty arthritis and uric acid kidney stones, respectively. The pharmacological interventions used widely for the prevention of gout involve drugs such as allopurinol and febuxostat. These drugs achieve the desired effect by inhibiting xanthine oxidase, which limits the conversion of purine products and eventually leads to decreased production of uric acid within the body and limitation of gout symptoms.[9]

Additionally, xanthine oxidase has inhibitory effects on certain drugs. These include the antimetabolite class of cancer drugs like azathioprine and 6-mercaptopurine, both of which are metabolized by xanthine oxidase. Concurrent use of XO inhibitors and anti-cancer drugs leads to excess anti-cancer drug accumulation in the body due to the limited breakdown, and subsequent dose reductions then become mandatory. If there is poor dose and drug regimen management, anti-cancer drug overdose, and associated toxic adverse effects can occur.[10]

As aforementioned above, higher activity of xanthine oxidase correlates with an elevated risk of cardiovascular disease along with other pathologies. As such, it has been postulated and should be further studied, that patients on xanthine oxidase inhibitors such as allopurinol who display decreased enzymatic activity, can potentially have lowered risks of cardiovascular and other pathological conditions. This effect can be tied to liver diseases as well as the presence of xanthine oxidase in greater quantities has also been seen in patients with severe liver injury. Although the reason for the correlation is not definitive as of yet, expectations are that the greater the severity of liver diseases, the higher the levels of xanthine oxidase will be in the serum. There remain more specific tests to evaluate liver function, but xanthine oxidase levels can also be an indicator of hepatic health.[11]