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

Rabbit brush-border membrane vesicles possess ferricyanide reducing activit y. This activity is preferentially dependent on NADH as reductant, and can be stimulated by the addition of FMN. The latency of activity observed foll owing vesicle solubilisation suggests that the responsible component is tra nsmembranous, and partially sequestered on the inner-face of the vesicles p rior to full solubilisation. Subsequent increases in detergent concentratio n (>0.3% w/v lauryl maltoside),were found to be inhibitory. Ferricyanide re ducing activity was effectively inhibited by the sulphydryl modifying reage nts N-ethyl malemide and p-chloromercuribenzoate, but not by the flavin ana logue diphenylene iodonium, The ferric-reducing activity co-purified with a b-type haem when applied to Sephacryl S-200 columns. The putative cytochro me was found to be immunologically distinct from neutrophil cytochrome b(55 8).