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.  Nicotinamide adenine dinucleotide (NAD + ), a derivative of vitamin B 3 ( 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.  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.
There are four members of the EP receptor family, EP 1 (EP1), EP 2 (EP2), EP 3 (EP3), and EP 4 (EP4). The EP1 receptor is encoded by the PTGER1 gene located on chromosome and is composed of 3 exons that encode a protein of 402 amino acids. The EP2 receptor is encoded by the PTGER2 gene located on chromosome and is composed of 2 exons that encode a protein of 358 amino acids. The EP3 receptor is encoded by the PTGER3 gene located on chromosome and is composed of 11 exons that generate at least 8 alternatively spliced mRNAs. The EP4 receptor is encoded by the PTGER4 gene located on chromosome and is composed of 7 exons that encode a protein of 488 amino acids. The EP2 and EP4 receptors are coupled to the activation of a G s -type G-protein, the EP1 receptor is coupled to the activation of a G q -type G-protein, and the EP3 receptor has been shown to activate both G q - and G i -type G-proteins.