Inactivation of aconitase in yeast exposed to oxidative stress.

Inactivation of aconitase by oxidative stress was analyzed under the in vivo and in situ conditions of yeast cells. Treatment of yeast cells with paraquat caused a specific inactivation of aconitase without affecting the activity of other citric acid cycle enzymes. Addition of copper plus ascorbic acid to permeabilized yeast cells also inactivated aconitase, but did not affect other TCA cycle-related enzymes. Inactivation of aconitase was suggested to be due to the superoxide and hydroxyl radicals produced from the reaction of O2 with paraquat and by Fenton reaction with copper and ascorbic acid under the in vivo and in situ conditions of yeast, respectively. Citrate the substrate of aconitase effectively protected aconitase from the oxidative inactivation. Toxicity of oxygen to yeast cells can be explained by the specific inactivation of aconitase by oxygen radicals. Increased concentrations of citrate can act as a defense mechanism against oxidative inactivation of aconitase under the exposure of aerobically grown yeast to oxidative stress.