Taurine

Taurine is a naturally occurring organic molecule. It is very common in the body of humans, making up about one-tenth of one percent of the weight of the body. It is found in high concentrations in bile and lower concentrations in the large intestine. It's biological role is complex. It is involved in the regulation of bile, preventing damage from oxygen, regulating osmosis, stabilizing membranes and modulating the calcium feedback cycle. It is also involved in the formation of skeletal muscle, the retina and the central nervous system.
Taurine can be commercially synthesized, often for use in pharmaceuticals, but also for use in pet food where it is an important nutritional supplement. It is found naturally in meat and fish. However, most mammals can synthesize taurine from other dietary sources; one of the animals that can't are felines, ostensibly because their all meat diet would make it unnecessar. Vegans have lower levels of taurine.
Taurine at Wikipedia