Effects of pH and temperature on the reaction of milk xanthine oxidase with 1-methylxanthine

Abstract

The reaction of milk Xanthine Oxidase (XO) with 1-methylxanthine has been investigated by steady state and stopped flow transient kinetic studies to understand the effect of a methyl group substituent on the purine ring. The pH dependence of the steady state kinetic parameter (V_max/K_m) shows a bell-shaped curve implying at least two ionisable groups are involved in the binding of XO with 1-methylxanthine, the higher pK_a 7.7 corresponding to ionisation of the substrate and the lower one 6.2 to the enzyme (possibly Glu-1261 by analogy to Glu-869 of aldehyde oxidoreductase from Desulfovibrio gigas). The temperature dependence of the steady state and transient kinetic studies suggests the existence of at least one molecular intermediate during breakdown of the enzyme–substrate complex. The thermodynamic parameters of the microscopic rate constants were determined from the temperature dependence studies.