COMPETITION BETWEEN INHIBITORS OF THE TRYPANOSOME ALTERNATIVE OXIDASE(TAO) AND REDUCED COENZYME Q(9)

The trypanosome alternative oxidase (TAO) is an attractive target for chemotherapy for the diseases caused by African trypanosomes because t here is no equivalent enzyme in mammalian hosts. Many inhibitors of th is enzyme have been described, but there have been no data on the mech anism of inhibition. In the present study, reduced 2,3-dimethoxy-5-met hyl-6-decyl-1,4-benzoquinone (decyl-CoQ-H-2) was used as a substitute for the natural substrate CoQ(9)-H-2 to allow direct measurements of t he TAO in crude mitochondrial preparations from Trypanosoma brucei bru cei. A K-m value of 3.8 mu M was obtained for this substrate. The foll owing five compounds that have alkyl side chains from 1 to 4 carbons a nd belong to three classes of inhibitors showed a competitive inhibiti on pattern with respect to decyl-CoQ-H-2: p-methoxybenzhydroxamic acid , p-ethoxybenzhydroxamic acid, p-n-butyloxybenzhydroxamic acid, methyl 3,4-dihydroxybenzoate and N-n-butyl-3,4-dihydroxybenzamide. The follo wing four compounds belonging to the same chemical classes but having alkyl side chains from 10 to 12 carbons showed uncompetitive inhibitio n patterns: p-n-dodecyloxybenzhydroxamic acid, n-decyl 3,4-dihydroxybe nzoate, n-dodecyl 3,4-dihydroxybenzoate, and N-n-decyl-3,4-dihydroxybe nzamide. Clearly, the first group of inhibitors compete with CoQ-H-2 f or the active site of the TAO. We propose that the uncompetitive patte rns produced by the second group of inhibitors are due to the greater lipophilicity of these compounds and the resulting change in the inter action of the inhibitors and the membrane containing the TAO, thus aff ecting the local concentration of the inhibitors at the active site.