Antibacterial peptide from normal rabbit serum. 3. Inhibition of microbial electron transport

The influence of the primary rabbit serum bactericide, PC-III, on the respiratory activity of Bacillus subtilis has been examined. Glucose- or lactate-dependent respiration by whole cells was rapidly and completely inhibited by concentrations of the bactericide producing significant cell death. Similar results were observed with membrane vesicles oxidizing NADH. In both cases, bactericide-induced inhibition of respiration was calcium dependent and blocked electron transport between cytochromes b and a. PC-III competed with oxidized Saccharomyces cytochrome c when the latter was used as an electron acceptor in cytochrome c reductase reactions catalyzed by B. subtilis membrane vesicles. Competitive inhibition by PC-III was also observed when reduced Saccharomyces cytochrome c was used as electron donor in the cytochrome c oxidase reaction. At an ionic strength of 0.13, PC-III exhibits a Ki of 25.9 and 102 nM for the reductase and oxidase complexes, respectively. Increasing the ionic strength to that producing optimal antibacterial action against whole cells (0.24) increased the Ki of PC-III for the reductase (75.4 nM), while the oxidase decreased (92.3 nM).