Autolysates of brewer's yeast contain several isoenzymes of fumarate reductase. One group (Type I) has a molecular weight of 62,000 to 63,000, as determined by chromatography on Sephadex, and 5 components in the group have been detected and partially or completely separated on triethylaminoethyl cellulose. Another group (Type II) consists of at least 2 isoenzymes. The predominant component shows a molecular weight of 34,000 and the minor one 112,000 ± 10,000 on Sephadex. One of the Type I enzymes has been extensively purified until it shows a single component on gradient centrifugation, although it appears to be heterogeneous on polyacrylamide electrophoresis. The prosthetic group is FAD and, in addition, iron and possibly, copper appear to be present. In contrast to mitochondrial succinate dehydrogenase the FAD is not covalently bound and the non heme iron is not linked to labile sulfide. Preliminary electron paramagnetic resonance data suggest that the iron may undergo oxidation‐reduction during the catalytic cycle. The absorption spectrum manifests the 450 mμ band of flavoproteins but there is an anomalous absorption below 400 mμ and in the long wave‐length region. Dithionite and succinate bleach the 450 mμ band and fumarate partially restores the color of the enzyme.
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