Redoxal as a new leadstructure for dihydroorotate dehydrogenase inhibitors: a kinetic study of the inhibition mechanism

Mitochondrial dihydroorotate dehydrogenase (DHO-dehase; EC 1.3.99.11) is a target of anti-proliferative, immuno-suppressive and anti-parasitic agents. Here, redoxal, (2,2'-[3,3'dimethosy[1,1'-biphenyl]-4,4'-diyl)diimino]bis-b enzoic acid, was studied with isolated mitochondria and the purified recomb inant human and rat enzyme to find out the mode of kinetic interaction with this target. Its pattern of enzyme inhibition was different from that of c inchoninic, isoxazol and naphthoquinone derivatives and was of a non-compet itive type for the human (K-ic = 402 nM; K-iu = 506 nM) and the rat enzyme (K-ic =116 nM; K-iu = 208 nM). The characteristic species-related inhibitio n of DHOdehase found with other compounds was less expressed with redoxal. In human and rat mitochondria, redoxal did not inhibit NADH-induced respira tion, its effect on succinate-induced respiration was marginal. This was in contrast to the sound effect of atovaquone and dichloroallyl-lawsone, stud ied here for comparison, In human mitochondria, the IC50 value for the inhi bition of succinate-induced respiration by atovaquone was 6.1 mu M and 27.4 mu M;I for the DHO-induced respiration; for dichlorallyl-lawsone, the IC50 values were 14.1 mu M and 0.23 mu M (C) 2000 Federation of European Bioche mical Societies.