INVOLVEMENT OF MAMMALIAN LIVER CYTOSOLS AND ALDEHYDE OXIDASE IN REDUCTIVE METABOLISM OF ZONISAMIDE

Zonisamide (1,2-benzisoxazole-3-methanesulfonamide) an anticonvulsant agent, is primarily metabolized to 2-sulfamoylacetyl-phenol by reducti ve cleavage of the 1,2-benzisoxazole ring. Rabbit liver cytosol with a n electron donor of aldehyde oxidase exhibited a significant zonisamid e reductase activity that was sensitive to inhibition by menadione, an inhibitor of aldehyde oxidase, The result suggested that the cytosoli c activity is caused by aldehyde oxidase, a cytosolic enzyme. In fact, rabbit and rat liver aldehyde oxidase had the ability to reduce zonis amide when supplemented with its electron donor, Apparent K-M and V-ma x values of aldehyde oxidase for zonisamide were 217 mu M and 42 nmol/ 10 min/mg protein in the case of the rabbit liver enzyme, and 542 mu M and 382 nmol/10 min/mg protein in the case of the rat liver enzyme, r espectively. In rabbits, hamsters, mice, and guinea pigs, zonisamide r eductase activity of the liver cytosols with 2-hydroxypyrimidine, an e lectron donor of aldehyde oxidase, was much higher than that of the li ver microsomes with NADPH. In rats, zonisamide reductase activity was examined with liver microsomes and cytosols from seven strains. The 2- hydroxypyrimidine-dependent cytosolic activity exhibited marked strain differences, unlike the NADPH-dependent microsomal activity, 1,2-Benz isoxazole was also reduced to salicylaldehyde by rabbit liver cytosol and aldehyde oxidase in the presence of 2-hydroxypyrimidine, Stoichiom etric studies showed that 2-sulfamoylacetylphenol was formed accompany ing nearly equimolar ammonia from zonisamide.