RHODOQUINONE AND COMPLEX-II OF THE ELECTRON-TRANSPORT CHAIN IN ANAEROBICALLY FUNCTIONING EUKARYOTES

Many anaerobically functioning eukaryotes have an anaerobic energy met abolism in which fumarate is reduced to succinate. This reduction of f umarate is the opposite reaction to succinate oxidation catalyzed by s uccinate-ubiquinone oxidoreductase, complex II of the aerobic respirat ory chain. Prokaryotes are known to contain two distinct enzyme comple xes and distinct quinones, menaquinone and ubiquinone (Q), for the red uction of fumarate and the oxidation of succinate, respectively. Paras itic helminths are also known to contain two different quinones, Q and rhodoquinone (RQ). This report demonstrates that RQ was present in al l examined eukaryotes that reduce fumarate during anoxia, not only in parasitic helminths, but also in freshwater snails, mussels, lugworms, and oysters. It was shown that the measured RQ/Q ratio correlated wit h the importance of fumarate reduction in vivo. This is the first demo nstration of the role of RQ in eukaryotes, other than parasitic helmin ths. Furthermore, throughout the development of the liver fluke Fascio la hepatica, a strong correlation was found between the quinone compos ition and the type of metabolism: the amount of Q was correlated with the use of the aerobic respiratory chain, and the amount of RQ with th e use of fumarate reduction. It can be concluded that RQ is an essenti al component for fumarate reduction in eukaryotes, in contrast to prok aryotes, which use menaquinone in this process, Analyses of enzyme kin etics, as well as the known differences in primary structures of proka ryotic and eukaryotic complexes that reduce fumarate, support the idea that fumarate-reducing eukaryotes possess an enzyme complex for the r eduction of fumarate, structurally related to the succinate dehydrogen ase-type complex II, but with the functional characteristics of the pr okaryotic fumarate reductases.