Quenching of protein fluorescence by transient intermediates in the liver alcohol dehydrogenase reaction

The addition of saturating concentrations of NAD-+ and alcohol to liver alcohol dehydrogenase in a stopped flow fluorimeter results in a triphasic quenching of enzyme fluorescence. A rapid quenching occurs with a rate constant of 300 to 500 s-minus 1, followed by a slower reaction at 50 to 100 s-minus 1, and ultimately followed by a very slow reaction. The addition of NAD-+ to enzyme in the absence of substrate causes a rapid quenching of enzyme fluorescence at 300 to 500 s-minus 1, with the same amplitude as the rapid phase in the presence of substrate. These studies demonstrate that NAD-+ binding to liver alcohol dehydrogenase causes a conformational change at a rate compatible with the previously reported rate constant for proton release, indicating that proton release is probably coupled to the conformational change.