The synthesis of melanins occurs only in pigment-producing cells called melanocytes . In these cells, melanins are synthesized from tyrosine by a sequence of several reactions. A defect in this pathway causes the disorder known as albinism . Affected individuals, who are referred to as albinos, lack pigmentation since they cannot convert tyrosine to melanins. GABA appears to be an inhibitory transmitter in the brain and spinal cord.
Histidine is the precursor of histamine.
Functions of histamine Histamine is a potent vasodilator and may be a neural transmitter. It is released during allergic reactions. Antihistamine drugs are compounds that bear a structural similarity to histamine and can prevent the physiologic changes that it normally produces. Histidine decarboxylase catalyzes the formation of histamine from L-histidine.