Allopurinol metabolic conversion products and xanthine accumulation in allopurinol-treated plants

Abstract

Allopurinol metabolic products and xanthine accumulation were determined in allopurinol-treated bean, broad bean and wheat plants. The results obtained demonstrate that allopurinol applied through roots undergoes two main transformations in the plants: (i) xanthine oxidase-mediated oxidation to oxipurinol and (ii) ribosyl transfer reactions with consequent synthesis of ribonucleoside derivatives of both allopurinol and oxipurinol. In bean and broad bean leaves ribosidation prevailed over oxidation, whereas in wheat leaves ribosidation conversion was negligible while xanthine oxidase-mediated oxidation was greatly in excess. Therefore, the allopurinol absorbed by wheat was almost totally available for xanthine oxidase inhibition in the plant, while in bean and broad bean, due to the high conversion to ribonucleoside which does not inhibit the enzyme, only part was available for this purpose. Consequently, in wheat leaves the same rate of xanthine accumulation, a consequence of in vivo xanthine oxidase inhibition, was achieved with concentrations about 5 times lower than in bean and broad bean applied through roots. These and previous results in tobacco plants are the first report on in vivo metabolic ribosidation of allopurinol and oxipurinol in plants. The present results are discussed in relation to the already reported allopurinol-mediated biotrophical rust fungi growth inhibition and support the proposition that this phenomenon is connected mainly with in vivo allopurinol-mediated host xanthine oxidase inhibition.