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Many anaerobically functioning eukaryotes have an anaerobic energy metabolism in which fumarate is reduced to succinate. This reduction of fumarate is the opposite reaction to succinate oxidation catalyzed by succinate-ubiquinone oxidoreductase, complex II of the aerobic respiratory chain. Prokaryotes are known to contain two distinct enzyme complexes and distinct quinones, menaquinone and ubiquinone (Q), for the reduction of fumarate and the oxidation of succinate, respectively. Parasitic helminths are also known to contain two different quinones, Q and rhodoquinone (RQ). This report demonstrates that RQ was present in all 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 with the importance of fumarate reduction in vivo. This is the first demonstration of the role of RQ in eukaryotes, other than parasitic helminths. Furthermore, throughout the development of the liver fluke Fasciola hepatica, a strong correlation was found between the quinone composition 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 the use of fumarate reduction. It can be concluded that RQ is an essential component for fumarate reduction in eukaryotes, in contrast to prokaryotes, which use menaquinone in this process. Analyses of enzyme kinetics, as well as the known differences in primary structures of prokaryotic and eukaryotic complexes that reduce fumarate, support the idea that fumarate-reducing eukaryotes possess an enzyme complex for the reduction of fumarate, structurally related to the succinate dehydrogenasetype complex II, but with the functional characteristics of the prokaryotic fumarate reductases.Living with hypoxia or even anoxia is an everyday experience for many organisms. Not only many prokaryotes, but many eukaryotic organisms as well can function (temporarily) without oxygen. Parasitic helminths, freshwater snails, and some lower marine organisms are known to be able to survive anaerobic conditions by adaptation of their energy metabolism. In addition to simple fermentation in which glucose is degraded to ethanol or lactate, most of these facultative anaerobic eukaryotes contain another fermentation variant, malate dismutation (Fig. 1). Malate dismutation is found in both strictly and facultative anaerobically functioning prokaryotes as well as in some eukaryotes that are capable of functioning anaerobically, like parasitic helminths (1), freshwater snails (2), mussels (3), oysters (4), and lugworms and other marine invertebrates (5). Although several variations of malate dismutation with various end products occur, the use of the production of succinate as an electron sink is universal. The reduction of malate to succinate occurs in two reactions that reverse part of the Krebs cycle, and the reduction of fumarate is the essential NADHconsuming reaction to maintain redox balance. There...

The major histocompatibility complex(MHC)-linked fourth component of complement (C4) shows a high degree of polymorphism in several animal species. In man C4 polymorphism was detected by distinct charge differences of the variants. O'Neill et al. showed that this C4 polymorphism was controlled by two closely linked genetic loci, F (C4A) and S (C4B) and these results were extended by Awdeh et al. with an improved typing method. Biochemical analysis of human C4 has revealed that it consists of three polypeptide chains, alpha, beta and gamma. In all reports so far on the molecular analysis of human C4, no molecular weight differences between the A and B locus-encoded molecules have been noticed. Here we demonstrate that the C4A and C4B locus-encoded alpha-chains have a molecular weight (MW) of 96,000 and 94,000, respectively, presenting for the first time a molecular basis for the difference between all C4A and C4B variants tested. Even rare variants that are difficult to allocate to the A or B locus on the basis of charge differences could be identified as C4A or C4B variants in this way, thereby providing new insights into the relationships between the C4A and C4B loci.

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