A vitamin is an organic compound needed in small quantities that cannot be made in cells. In human nutrition, most vitamins function as coenzymes after modification; for example, all water-soluble vitamins are phosphorylated or are coupled to nucleotides when they are used in cells.^[18] Nicotinamide adenine dinucleotide (NAD⁺), a derivative of vitamin B₃ (niacin), is an important coenzyme that acts as a hydrogen acceptor. Hundreds of separate types of dehydrogenases remove electrons from their substrates and reduce NAD⁺ into NADH. This reduced form of the coenzyme is then a substrate for any of the reductases in the cell that need to reduce their substrates.^[19] Nicotinamide adenine dinucleotide exists in two related forms in the cell, NADH and NADPH. The NAD⁺/NADH form is more important in catabolic reactions, while NADP⁺/NADPH is used in anabolic reactions.