Nicotinamide adenine dinucleotide ( NAD ) is a coenzyme found in all living cells. The compound is a dinucleotide, because it consists of two nucleotides joined through theirphosphate groups. One nucleotide contains an adenine base and the other nicotinamide. Nicotinamide adenine dinucleotide exists in two forms: an oxidized and reduced form abbreviated as NAD+ and NADH respectively.