Uric acid

It is a minor end-product of nitrogen metabolism in the human body (the main product being urea), and is found in small amounts in urine.
In some other animals, such as birds and reptiles, it is the main end-product, and is excreted in feces. The high nitrogen content of uric acid is why guano is so valuable as a fertiliser in agriculture.

Unlike urea and ammonia, uric acid can be excreted as a dry solid. While this compound is more energetic than those other wastes, it means that water loss due to elimination can be reduced to a minimum. It is therefore commonly found in the excretions of animals that live in very dry environments. Most birds eliminate nitrogen in the form of uric acid, and the crystalline form of the compound is used as a reflector in certain species of fireflies.

In human blood, uric acid concentrations between 3.6 and 8.3 mg/dL are considered normal by the American Medical Association, although significantly lower levels are common in vegetarians.

The disease gout in humans is associated with abnormal levels of uric acid in the system. Saturation of uric acid in the human blood stream may result in one form of kidney stones when the acid crystallizes into solid inside the kidney. A percentage of gout patients eventually get uric kidney stones.

see also: xanthine oxidase