Physiological role of rhodoquinone in Euglena gracilis mitochondria

Abstract

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 UQ 9. 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.