Urate oxidation by cupric ion (Cu⁺⁺)

Abstract

Uric acid or urate, a compound of extremely limited solubility produced in vivo by degradation of purines from nucleic acids or other endogenous and exogenous sources -has long been considered a metabolic "enfant terrible" of man. Possessing a vestigial uricotelic apparatus but lacking the copper-containing enzyme uricase which oxidizes urate to water-soluble compounds, man teters on the brink of urico-disaster. Although elaborate control mechanisms exist to prevent flooding by urate, when these systems fail for genetic or other reasons body urate pools increase and there is danger of precipitation in soft tissues, cartilage (tophi) and urine (stones). While the major portion (60-70%) of the urate disappearing from the urate pool is excreted as such in urine, the remainder may be converted to CO,, allantoin, allantoic acid, urea, and NH, by bacteria of the gastrointestinal tract [ 1 J . In addition oxygen uptake studies have revealed that urate may be oxidized by Cu++ above pH 10 and by the cytochrome oxidase system (Cu++?) at pH 7.9 [2]. Hemoproteins in the presence of peroxide causes breakdown of urate to allantoin, CO, and alloxanic acid [3].We are reporting in vitro urate oxidation mediated solely by Cu++ at pH 6.0-8.2. In fig. 1 the fall in absorption of a urate solution incubated with Cu++ (urate/Cu++ ratio 1:4) is shown. Cu++ also alters the shape of the urate absorption spectrum drastically, obliterating the second maximum at 237 rnp. H,S stops the fall in W absorption caused by Cu++, the centrifuged supernatant fluid has the same absorption spectrum as urate. When uric acid-6-14C was incubated with Cu++ in a closed vessel, labeled CO, was evolved. This was also the case when Cu+ and H202 339

Uricase from leaves: its purification and characterization from three different higher plants

“…Data (in kDa) were obtained by electrophoresis according to Hedrick and Smith (1968) common feature of uricases (Vogels and van der Drift 1976;Bongaerts et al 1978;Machida and Nakanishi 1980;Rainbird and Atkins 1981;Alamillo et al 1991) but its mechanism is not well understood. However, we did not ®nd inhibition, but a slight stimulation (5%) in all the cases (Table 3), which could be due to the ability of cupric ions to catalyze urate oxidation (Schein and Kunin 1969). Since immunological speci®city is a useful tool with which to compare biological material from diverse sources, the ability of puri®ed plant uricases to recognize antibodies against other uricase was tested (Fig.…”

Degradation of purines and pyrimidines by microorganisms.

“…The assumed presence of copper is partly based on the following observations: (i) Urate oxidation to allantoin can be mediated solely by Cu2+ or by Cu2+ plus H202 at pH 6.0 to 8.2 (459) or at higher pH values in the presence of polyvalent anions or cyanide (40). Fe3+ was without effect (459). (ii) Cyanide exerts an inhibiting effect.…”

Kinetic studies of copper-induced oxidation of urate, ascorbate and their mixtures

“…For the sake of simplicity, not all the measured time points are included in the Figures Prior to the addition of copper, two peaks were observed at 230 and 292 nm, as demonstrated previously by Schein and Kunin [22] and by Lam et al [20]. When 5 lM Cu(II) was added to the urate solution, the absorbance declined with an apparent t 1/2 of less than 90 min, so that after about 180 min only a residual peak was monitored at 292 nm.…”

“…Accordingly, the major product of non-enzymatic oxidation of urate in serum, allantoin, is regarded a useful indicator of the individual oxidative stress [18,19] because it was found to be higher in the serum of patients suffering from diseases associated with oxidative stress [18,19]. Hence, non-enzymatic metal-catalyzed oxidation of urate in vivo may account for degradation of urate in organisms that lack uricase, including humans [3,[20][21][22].…”