Aerobic electron transport in Propionibacterium shermanii. Effects of cyanide

Cyanide inhibited D- and L-lactate and NADH oxidase activities of membrane particles from Propionibacterium shermanii but only at relatively high concentrations. Inhibition occurred at two different sites in the electron transport pathway. One site, with a half-maximal inhibition concentration (I0.5) of 2 to 3 mM KCN, is located at the terminal oxidase involved in cytochrome b oxidation; the evidence is consistent with cytochrome d being the major oxidase involved. At high concentrations, cyanide inhibited reduction of cytochrome b by D-lactate (I0.5 value 20-25 mM cyanide). A proportion of the oxygen-uptake remained uninhibited even by 100 mM cyanide; this proportion was about 80% for succinate, 30% for L-lactate, 15% for D-lactate and 10% for NADH. The oxygen uptake per mol of substrate oxidised increased with increasing cyanide concentration and was accompanied by the formation of hydrogen peroxide as a product of a cyanide-insensitive oxidase system.