COMPARATIVE-STUDY AND CDNA CLONING OF THE FLAVOPROTEIN SUBUNIT OF MITOCHONDRIAL COMPLEX-II (SUCCINATE-UBIQUINONE OXIDOREDUCTASE - FUMARATE REDUCTASE) FROM THE DOG HEARTWORM, DIROFILARIA-IMMITIS

Mitochondrial complex II functions as a fumarate reductase (FRD), the reverse reaction of succinate dehydrogenase (SDH), and plays an import ant role in the anaerobic respiratory chain of parasitic helminths. In this study, complex II from the dog heartworm, Dirofilaria immitis ad ult, which is thought to act as a homolactatic fermenter, was examined in terms of its enzymatic features and primary structure in order to investigate the possible role of mitochondria in this filaria. Mitocho ndria from D. immitis adult showed high FRD activity when the enzymati c assay was performed using methylviologen as an artificial electron d onor, The ratio of SDH to FRD in D. immitis was comparable to that in Ascaris suum adult, which is known to have an anaerobic mitochondrial respiratory chain with a high FRD activity of complex II, The FRD acti vity of D. immitis mitochondria was inhibited by the sulfhydryl reagen t N-ethylmaleimide (NEM), while that of A. suum complex II was resista nt to this inhibitor, The presence of the flavoprotein (Fp) subunit, w hich contains the substrate binding active site, was confirmed in D. i mmitis mitochondria by immunoblotting using a monoclonal antibody agai nst the A. suum Fp subunit, By homology probing with the polymerase ch ain reaction, the entire cDNA for the D. immitis adult Fp was cloned a nd sequenced, The deduced amino acid sequence showed significant homol ogy to that of A. suum and other mitochondrial Fps, in contrast to muc h less similarity to bacterial FRD, even though the D. immitis complex II showed high FRD activity, These results are the first indication o f the presence of a functional complex II in D. immitis mitochondria,