The ferric-reducing activity of duodenal brush-border membrane vesicles is associated with a b-type haem

Rabbit brush-border membrane vesicles possess ferricyanide reducing activity. This activity is preferentially dependent on NADH as reductant, and can be stimulated by the addition of FMN. The latency of activity observed following vesicle solubilisation suggests that the responsible component is transmembranous, and partially sequestered on the inner-face of the vesicles prior to full solubilisation. Subsequent increases in detergent concentration (> 0.3% w/v lauryl maltoside) were found to be inhibitory. Ferricyanide reducing activity was effectively inhibited by the sulphydryl modifying reagents N-ethyl malemide and p-chloromercuribenzoate, but not by the flavin analogue diphenylene iodonium. The ferric-reducing activity co-purified with a b-type haem when applied to Sephacryl S-200 columns. The putative cytochrome was found to be immunologically distinct from neutrophil cytochrome b558.