Effects of allopurinol on endotoxin-induced increase in serum xanthine oxidase in the horse

Using a modified bovine milk enzyme kinetic assay, xanthine oxidase activity of serum collected from 34 adult, healthy horses of both sexes was determined. Enzyme activity varied from 0 to 126 mU litre−1 with a mean of 44·95 ± 21·05 mU litre−1. The optimal pH and temperature for maximal activity were 7·8 and 28°C, respectively. Freezing the serum for four days at −70°C did not destroy the enzyme activity. Various doses (25, 50 and 75 μg kg−1, intraperitoneally) of endotoxin (lipopolysaccharide D1Escherichia coli O26:B6) previously known to have caused moderate to severe systemic clinical signs of endotoxaemia in horses produced a significant dose related increase in serum xanthine oxidase activity. Pretreatment (12 hours) with allopurinol (5 and 50 mg kg−1, intraperitoneally) significantly reduced the rise in xanthine oxidase activity in endotoxin (50 μg kg−1, intraperitoneally) treated horses. The results of this study suggest that xanthine oxidase catalysed production of superoxide radicals may play a role in the pathogenesis of endotoxaemia and that allopurinol, an alternate substrate, should be further evaluated for its therapeutic potential in endotoxin related systemic diseases in horses.