Both monoferric forms of OTf, each of about 80 kDa, bound to CERBC enough tightly, but at a lesser extent with respect to Fe2OTf with a Bmax in the order: 59Fe2OTf greater than OTf59FeC much greater than 59FeNOTf. 2. Fe2OTf competed, in equimolar ratio, with 59FeNOTf or OTf59FeC, lowering the Bmax value; a 10-fold molar excess of Fe2OTf almost abolished the binding of both labelled monoferric forms. Apo-OTf did not compete with the monoferric forms for binding to CERBC. Iron-saturated N- or C-terminal OTf half-molecules, each of about 40 kDa, were unable to displace the monoferric form. By contrast, the mixture of both half-molecules gave results very similar to Fe2OTf. 59FeNOTf and OTf59FeC were displaced by a molar excess of both unlabelled monoferric forms. 3. Uptake experiments showed that both monoferric forms of OTf were less effective in delivering iron to CERBC with respect to the diferric form, but, nevertheless, there was still an appreciable iron uptake which paralleled the binding behaviour, being the C-form slightly more efficient than the N-form.
