The effects of 8-azaguanine and 6-mercaptopurine on purine catabolism in the rat.

Various purine analogs active as carcinostatic agents have previously been shown to act as po tent inhibitors of xanthine oxidase in vitro in both homogenate and isolated enzyme systems (4, 5). 8-Azaguanine (8-AG) has been demonstrated to be an in vitro inhibitor of both xanthine oxidase and uricase (4, 9). Elion et al. have reported that 6mercaptopurine (6-MP) is a substrate for xanthine oxidase and is oxidized to thiouric acid (3). It seemed possible, therefore, that 6-MP could com petitively interfere with the interaction of this enzyme with its physiologic substrate. It is the purpose of this study to determine whether these in vitro relationships are of exclusive interest to the enzymologist or whether they occur in vivo and therefore deserve the attention of those interested in ascertaining the mechanism of action of carcinostatic agents. With isotopically labeled compounds used to follow biochemical events in vivo, it is possible to elucidate the effects of drugs on these metabolic reactions. In rats, xanthine is oxidized by xanthine oxidase to uric acid, which is then further oxidized and decarboxylated by uricase to allantoin (Chart 1). In this reaction sequence, the number 6 carbon of xanthine is converted to CCV With xanthine-6C14,the rate at which the radioactive carbon atom appears in the expired COj provides a direct meas ure of xanthine catabolism and thus permits study of the in vivo effects of drugs on the sum of the xanthine oxidaseand uricase-catalyzed reactions. The use of uric acid-6-C14 permits elucidation of the in vivoeffects of drugs on the uricase-catalyzed reaction alone.