Coenzyme Q(10) (CoQ(10)) is an electron carrier in the respiration chain with antioxidant activity. The effects of aeration and agitation rates, dissolved oxygen levels, an electron flux inhibitor (azide), a proton gradient releaser (2,4-dinitrophenol (DNP)), and an oxidative stressor (hydrogen peroxide (H2O2)) on the intracellular CoQ(10) contents in Agrobacterium tumefaciens ATCC4452 were investigated. With a decrease of dissolved oxygen level from 20 to 5%, the intracellular CoQ(10) content increased about four-fold, yielding 2 mg/g dry cell weight at 5% dissolved oxygen level. Azide significantly increased the intracellular CoQ(10) content, with the highest value of 5.3 mg/g dry cell weight in the presence of 0.45 mM of sodium azide. However, DNP (up to 200 mu M) and H2O2 (up to 10 mu M) did not affect the intracellular CoQ(10) content, indicating proton gradient release, and oxidative stress do not affect the synthesis of CoQ(10). These results show that restricted electron flux by limited oxygen supply and the addition of azide increases the intracellular CoQ(10) content, suggesting a feedback regulation of CoQ(10) biosynthesis by its physiological function of electron carrier in the respiration chain. (c) 2005 Elsevier Ltd. All rights reserved.