Calcium oxalate stone disease: role of lipid peroxidation and antioxidants
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Membrane injury facilitated the fixation of calcium oxalate crystals and subsequent growth into kidney stones. Oxalate-induced membrane injury was mediated by lipid peroxidation reaction through the generation of oxygen free radicals. In urolithic rat kidney or oxalate exposed cultured cells, both superoxide anion and hydroxyl radicals were generated in excess, causing cellular injury. In hyperoxaluric rat kidney, both superoxide and H2O2-generating enzymes such as glycolic acid oxidase (GAO) and xanthine oxidase (XO) were increased, and hydroxyl radical and transition metal ions, iron, and copper were accumulated. The lipid peroxidation products, thiobarbituric acid-reactive substances (TBARS), hydroperoxides, and diene conjugates were excessively released in tissues of urolithic rats and in plasma of rats as well as stone patients. The accumulation of these products was concomitant with the decrease in the antioxidant enzymes, superoxide dismutase (SOD), catalase, glutathione peroxidase (GPx), and glucose-6 phosphate dehydrogenase (G6PD) as well as radical scavengers, vitamin E, ascorbic acid, reduced glutathione (GSH), and protein thiol. All the above parameters were decreased in urolithic condition, irrespective of the agents used for the induction of urolithiasis. Oxalate binding activity and calcium oxalate crystal deposition were markedly pronounced, along with decreased adenosine triphosphatase (ATPase) activity. Lipid peroxidation positively correlated with cellular oxalate, oxalate binding, γ-glutamyl carboxylase, and calcium level and negatively correlated with GSH, vitamin E, ascorbic acid, and total protein thiol. Antioxidant therapy to urolithic rats with vitamin E, glutathione monoester, methionine, lipoic acid, or fish oil normalised the cellular antioxidant system, enzymes and scavengers, and interrupted membrane lipid and protein peroxidation reaction, ATPase inactivation, and its associated calcium accumulation. Antioxidant therapy prevented calcium oxalate precipitation in the rat kidney and reduced oxalate excretion in stone patients. Similarly, calcium oxalate crystal deposition in vitro to urothelium was prevented by free radical scavengers such as phytic acid and mannitol by protecting the membrane from free radical-mediated damage. All these observations were suggestive of the active involvement of free radical-mediated lipid peroxidation-induced membrane damage in the pathogenesis of calcium oxalate crystal deposition and retention.
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