Physiological role of rhodoquinone in Euglena gracilis mitochondria

Rhodoquinone (RQ) participates in fumarate reduction under anaerobiosis in some bacteria and some primitive eukaryotes. Euglena gracilis, a facultative anaerobic protist, also possesses significant rhodoquinone-9 (RQ(9)) content. Growth under low oxygen concentration induced a decrease in cytochromes and ubiquinone-9 (UQ(9)) content, while RQ(9) and fumarate reductase (FR) activity increased. However, in cells cultured under aerobic conditions, a relatively high RQ(9) content was also attained together with significant FR activity. In addition, RQ(9) purified from E. gracilis mitochondria was able to trigger the activities of cytochrome bc(1) complex, bc(1)-like alternative component and alternative oxidase, although with lower efficiency (higher K-m, lower V-m) than UQ(9). Moreover, purified E. gracilis mitochondrial NAD(+)-independent D-lactate dehydrogenase (D-iLDH) showed preference for RQ(9) as electron acceptor, whereas L-iLDH and succinate dehydrogenase preferred UQ9. These results indicated a physiological role for RQ(9) under aerobiosis and microaerophilia in E. gracilis mitochondria, in which RQ(9) mediates electron transfer between D-iLDH and other respiratory chain components, including FR. (c) 2005 Elsevier B.V. All rights reserved.