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2,6,8-trioxypurine known as uric acid represents a heterocyclic derivative of purine, with a molecular weight of 158 Da. In contrast to primates and humans, in the other animals uric acid is further oxidized by the uricase enzyme (urate oxidase) to allantoin, 100 times more water-soluble than uric acid and, consequently, more efficient excreted in the urine. Humans and higher pri¬mates lack a functional uricase gene. The uricase activity loss in higher primates and humans occurred about 15 million years ago, re¬sulting in a relatively higher plasma uric acid level than that in lower ani¬mals. This was an impressive and very important evolutionary event, providing essential survival advantages for humans. Uric acid represents a powerful antioxidant capable of scavenging singlet oxygen molecules, oxygen radicals, and peroxynitrite (ONOO−) molecules. However, in vivo and cellular research studies are interestingly illustrating that, depending on its chemical microenvironment, the uric acid molecule plays a duplicate role, being also a pro-oxidant. Uric acid represents the most important salivary antioxidant, in the circumstances of very low salivary thiols concentrations. Uric acid plays an important role in oral pathology and may be regarded as a very useful future salivary biomarker. [ABSTRACT FROM AUTHOR]