The paradoxical cyanide-stimulated respiration of Zymomonas mobilis: cyanide sensitivity of alcohol dehydrogenase (ADH II)

The respiratory inhibitor cyanide stimulates growth of the ethanologenic bacterium Zymomonas mobilis, perhaps by diverting reducing equivalents from respiration to ethanol synthesis, thereby minimizing accumulation of toxic acetaldehyde. This study sought to identify cyanide-sensitive components of respiration. In aerobically grown, permeabilized Z. mobilis cells, addition of 200 μM cyanide caused gradual inhibition of ADH II, the iron-containing alcohol dehydrogenase isoenzyme, which, in aerobic cultures, might be oxidizing ethanol and supplying NADH to the respiratory chain. In membrane preparations, NADH oxidase was inhibited more rapidly, but to a lesser extent, than ADH II. The time-course of inhibition of whole-cell respiration resembled that of NADH oxidase, yet the inhibition was almost complete, and was accompanied by an increase of intracellular NADH concentration. Cyanide did not significantly affect the activity of ADH I, the zinc-containing alcohol dehydrogenase isoenzyme. When an aerobic batch culture was grown in the presence of 200 μM cyanide, cyanide-resistant ADH II activity was observed, its appearance correlating with the onset of respiration. It is concluded that the membrane-associated respiratory chain, but not ADH II, is responsible for the whole-cell cyanide sensitivity, while the cyanide-resistant ADH II is needed for respiration in the presence of cyanide, and represents an adaptive response of Z. mobilis to cyanide, analogous to the induction of alternative terminal oxidases in other bacteria.