The enzyme ADH from this case requires the co-factor Nicotinamide Adenine Dinucleotide (NAD+) in order to function. This co-factor takes up a hydride ion (H-) from the ethanol, leading to the formation of an acetaldehyde. NAD+ is reduced to NADH and thereby works as an electron acceptor (see figure 1). NAD+ is derived from the vitamin niacin. Without NAD+, ADH would not function.
Figure 1: The chemical structure of NAD+ and the manner in which it is reduced to NADH by the addition of two electrons and a proton.