Final answer:
The zero kinetics step in ETOH metabolism is the conversion of ethanol to acetaldehyde by the alcohol dehydrogenase system, proceeding at a constant rate characteristic of a zeroth-order reaction.
Step-by-step explanation:
The ETOH metabolism step that is zero kinetics is the initial oxidation of ethanol to acetaldehyde, catalyzed by the alcohol dehydrogenase system. At higher concentrations of ethanol, this step proceeds at a constant rate, which is characteristic of a zeroth-order reaction. This means that the reaction rate is independent of the concentration of ethanol itself and is limited by the enzyme capacity.
The alcohol dehydrogenase system is responsible for approximately 80 to 90% of the hepatic ethanol metabolism. In this process, not only is acetaldehyde produced, which is toxic, but also NADH and H+ ions. This reaction is crucial as it starts the breakdown of alcohol in the liver. In parallel, the microsomal ethanol oxidizing system (MEOS) also contributes to ethanol metabolism, particularly in heavy drinkers, which can clear ethanol faster.