INHIBITION OF HUMAN GLUTATHIONE-REDUCTASE BY 10-ARYLISOALLOXAZINES - CRYSTALLOGRAPHIC, KINETIC, AND ELECTROCHEMICAL STUDIES

A series of newly synthesized N-10-arylisoalloxazines-some of which ar e known to be antimalarial agents-were studied as inhibitors of human glutathione reductase (GR; NADPH + GSSG + H+ reversible arrow NADP(+) + 2GSH). The flavoenzyme was inhibited with IC50 values between less t han or equal to 1 and 100 mu M in the presence of 100 mu M GSSG and 10 0 mu M NADPH. The isoalloxazines and N-3-methylisoalloxazines with a 4 '-chlorophenyl or a 3',5'-dichlorophenyl group at N10 were found to be the most promising inhibitors of GR, although even the bulkier 10-nap hthyl and -anthryl derivatives were also effective inhibitors. In cont rast, at position N3 of the isoalloxazine ring, the size of the substi tuent was found to strongly influence the inhibitory effect. Introduct ion of a carboxymethyl group at N3-which markedly increased the solubi lity of the derivative in aqueous solutions-caused a rise in the IC50 values by 1 order of magnitude. 8-Fluoro- and 8-azido-10-arylisoalloxa zines were potent inhibitors of GR; consequently position C8 of the be nzenoid subnucleus instead of N3 should be considered for introducing substituents. No correlation was observed between the inhibitory stren gth of several isoalloxazines and their redox potential as measured by cyclovoltammetry. The crystallographic analysis of GR complexed with 0-(4'-chlorophenyl)-3-(carboxymethyl)isoalloxazine and 5'-dichlorophen yl)-3-(carboxymethyl)isoalloxazine, respectively, revealed the presenc e of one inhibitor molecule bound at the 2-fold axis of the homodimeri c protein. This location is consistent with fluorescence titration mea surements and enzyme kinetic studies in solution which gave-no indicat ion for binding at the substrate sites.