5-N-Carboxyimino-6-N-chloroaminopyrimidine-2,4(3H)-dione as a hypochlorite-specific oxidation product of uric acid

LC/MS/MS detection of uric acid oxidation metabolites to evaluate reactive oxygen and nitrogen species in vivo with application to pseudo-inflammation

Uric acid, a water-soluble antioxidant ubiquitous in human bodily fluids at relatively high concentrations, reacts with a variety of reactive oxygen and nitrogen species. Ours and other previous studies identified allantoin, oxaluric acid, triuret, and 5-N-carboxyimino-6-aminopyrimidine-2,4(3H)-dione as specific metabolites reactive against free radicals, singlet oxygen, peroxynitrite, and hypochlorous anion, respectively. We analyzed human plasma spiked with these products using high-performance liquid chromatography-tandem mass spectrometry. We observed recoveries of 40-110% and coefficients of variance within 7%. Samples remained stable at -80°C for at least 4 weeks, indicating the analytical method is sound. Detection of these metabolites in biological samples enables the identification of each species generated in vivo. We observed changes in the products in human blood during pseudo-inflammation induced by treatment with lipopolysaccharide. Levels of allantoin, oxaluric acid, triuret, and 5-N-carboxyimino-6-aminopyrimidine-2,4(3H)-dione increased after addition of lipopolysaccharide. The formation of singlet oxygen was confirmed by increased formation of Trp-derived cis-hydroxide ([2S,3aR,8aR]-3a-hydroxy-1,2,3,3a,8,8a-hexahydropyrrolo[2,3-b]indole-2-carboxylic acid) (cis-WOH). We believe our method will aid development of strategies to treat oxidative stress-associated diseases.

5-N-Carboxyimino-6-aminopyrimidine-2,4(3H)-dione, a novel indicator for hypochlorite formation

Uric acid is an adequate and endogenous probe for identifying reactive oxygen or nitrogen species generated in vivo because its oxidation products are specific to reacted reactive oxygen or nitrogen species. Recently, we identified 5-N-carboxyimino-6-N-chloroaminopyrimidine-2,4(3H)-dione as a hypochlorite-specific oxidation product. 5-N-carboxyimino-6-N-chloroaminopyrimidine-2,4(3H)-dione was anticipated to be a biomarker for hypochlorite production in vivo. However, while it was stable in aqueous solution at weak acidic and alkaline pH (6.0-8.0), it was unstable in human plasma. In this study, we found that 5-N-carboxyimino-6-N-chloroaminopyrimidine-2,4(3H)-dione rapidly reacted with thiol compounds such as cysteine and glutathione to yield 5-N-carboxyimino-6-aminopyrimidine-2,4(3H)-dione, which was stable in human plasma unlike 5-N-carboxyimino-6-N-chloroaminopyrimidine-2,4(3H)-dione. 5-N-carboxyimino-6-aminopyrimidine-2,4(3H)-dione was produced upon uric acid degradation during myeloperoxidase-induced uric acid oxidation and lipopolysaccharide-induced pseudo-inflammation in collected 2,4(3H)-dione has potential as a marker for hypochlorite production in vivo.

Decreased serum uric acid in patients with traumatic brain injury or after cerebral tumor surgery

Objectives:

To investigate changes in sUA in patients with TBI or patients after cerebral tumor surgery and the possible mechanism of these changes.

Methods:

This prospective cohort study enrolled patients with TBI or underwent cerebral tumor surgery at West China Hospital, China, from November 2014 to May 2018. Serum UA (sUA) levels, urine excretion, UA oxidant product allantoin and other clinical parameters were assessed.

Results:

100 patients were enrolled for analysis. sUA in patients with TBI or underwent cerebral tumor surgery started to decline from day 1 after injury or surgery compared to control. This decreasing trend continued from day 3 (143.2±59.3 μmol/L, 188.8±49.4 μmol/L vs 287.3±80.2 μmol/L, p<0.0001) until day 7. No difference in urinary UA excretion was found in the TBI group or cerebral tumor surgery group. Urine allantoin and the allantoin to sUA ratio of the TBI group decreased on day 3 compared with the control group. The structural equation model showed that the sUA level was related to the Glasgow coma score (GCS) (r=0.5383, p<0.0001), suggesting the potential association of UA with consciousness level, as well as serum protein and electrolytes including albumin, calcium and phosphate.

Conclusion:

The sUA was decreased in patients with TBI or underwent cerebral tumor surgery.

(2,5-Dioxoimidazolidin-4-ylidene)aminocarbonylcarbamic Acid as a Precursor of Parabanic Acid, the Singlet Oxygen-Specific Oxidation Product of Uric Acid

Previously, we identified that parabanic acid (PA) and its hydrolysate, oxaluric acid (OUA), are the singlet oxygen-specific oxidation products of uric acid (UA). In this study, we investigated the PA formation mechanism by using HPLC and a time-of-flight mass spectrometry technique and identified unknown intermediates as (2,5-dioxoimidazolidin-4-ylidene)aminocarbonylcarbamic acid (DIAA), dehydroallantoin, and 4-hydroxyallantoin (4-HAL). DIAA is the key to PA production, and its formation pathway was characterized using ¹⁸O₂ and H₂¹⁸O. Two oxygen atoms were confirmed to be incorporated into DIAA: the 5-oxo- oxygen from singlet oxygen and the carboxylic oxygen from water. Isolated DIAA and 4-HAL gave PA stoichiometrically. A plausible reaction scheme in which two pathways branch out from DIAA is presented, and the potential for PA as an endogenous probe for biological formation of singlet oxygen is discussed.