Characterization of the enzymatic activity for biphasic competition by guanoxabenz (1-(2,6-dichlorobenzylidene-amino)-3-hydroxyguanidine) at alpha2-adrenoceptors. I. Description of an enzymatic activity in spleen membranes

The mechanism for formation of high-affinity binding of 1-(2,6-dichlorobenzylidene-amino)-3-hydroxyguanidine (guanoxabenz) to alpha2-adrenoceptors was studied in particulate fractions from the rat spleen. The proportion of apparent high versus low-affinity alpha2-adrenoceptor binding sites increased with increasing incubation time and was also augmented by Mg2+ ions. The formation of high-affinity guanoxabenz binding seemed to be inhibited by a series of N-hydroxyguanidine analogs to guanoxabenz, as well as by a series of metabolic inhibitors that included allopurinol, 1-chloro-2,4-dinitrobenzene, 5,5'-dithiobis-(2-nitrobenzoic acid), cibacron blue, phenyl-p-benzoquinone, didox, and trimidox. The formation of guanoxabenz high-affinity binding was also inhibited in a time- and concentration-dependent fashion by preincubating the membranes with the LW03 N-hydroxyguanidine analogue of guanoxabenz. Moreover, when the spleen membranes were extensively washed for 30 min with buffers at 25 degrees, the guanoxabenz high-affinity binding disappeared. However, when these washed membranes were supplemented with xanthine, the apparent affinity of guanoxabenz increased four to five-fold. Taken together, all data were compatible with the theory that the formation of high-affinity binding was dependent on the generation of a guanoxabenz metabolite that showed an approximate 100-fold greater affinity for the alpha2-adrenoceptors than guanoxabenz itself. Because the most potent blocker of the formation of high-affinity binding was allopurinol (apart from some N-hydroxyguanidine analogs to guanoxabenz) and since the activity could be restored with xanthine, a likely candidate responsible for the metabolic activation is Xanthine Oxidase.