The sequence of the flavoprotein subunit of bovine heart succinate dehydrogenase

The cDNA sequence of the flavoprotein subunit of bovine heart succinate dehydrogenase is reported. This is the first complete eukaryotic sequence of the flavoprotein subunit to be characterized, and it encodes a 665-amino acid protein that consists of a presequence and a 621-residue mature protein. The deduced bovine sequence shows homology to the corresponding peptides of prokaryotic succinate dehydrogenase and the related fumarate reductases; in particular, there is good overall homology (48%) to the flavoprotein subunit of Escherichia coli succinate dehydrogenase. The conserved sequences comprising the active site and those involved in FAD binding are also found in the bovine protein. The active site of the bovine polypeptide contains a cysteine that confers sensitivity of the enzyme to sulfhydryl reagents; this cysteine is only present in some sequences and thus provides a discriminatory biochemical marker. A putative flavoprotein subunit of human placental succinate dehydrogenase (partial sequence) that lacks this critical cysteine (Malcovati, M., Marchetti, T., Zanelli, T., and Tenchini, M. L. (1991) in Flavins and Flavoproteins 1990 (Curti, B., Ronchi, S., and Zanetti, G., eds) pp. 727-730, Walter de Gruyter & Co., Berlin) has only 16% homology to the bovine heart flavoprotein subunit. However, we show that the enzyme from human placenta is as sensitive to N-ethylmaleimide as that from bovine tissues. In addition, a transcript in human placenta and muscle hybridizes to the bovine heart flavoprotein cDNA and is the same size as that in bovine tissues.