What They Saw
They isolated hydrogenases from cells but didn't separate them from the membrane, because that made them too sensitive to oxygen. When activated in a reducing environment and then exposed to NO, hydrogenase activity was inhibited, but this inhibition could be almost completely reversed by adding iron EDTA, which reacts with NO. The more NO, the more inhibition (relative to the no NO control).
When NO was added and the hydrogenase wasn't active, the inactivation was irreversible; the longer the exposure, the more the inhibition, but it took larger amounts of NO to get the same amount of inhibition as the reversible kind. Hydrogen or carbon monoxide didn't protect against this inactivation; hydrogen even enhanced the effect.