After urea is formed, it is excreted in the urine. An adult excretes 20–30 g of urea in the urine daily.
The overall equation of the urea cycle is:
Humans also excrete a second nitrogenous waste, uric acid. It is the product of nucleic acid, not protein, metabolism. It is produced within peroxisomes, and it is excreted in the urine, or reabsorbed by kidneys to regulate bloob levels. Uric acid is a potent antioxidant and thus can protect cells from damage by reactive oxygen species (ROS). In human blood plasma, the reference range of uric acid is typically 3.4–7.2 mg per 100 mL for men, and 2.4–6.1 mg per 100 mL for women. Uric acid concentrations in blood plasma above and below the normal range are known as, respectively, hyperuricemia and hypouricemia.
Uric acid is only slightly soluble in water and easily precipitates out of solution forming needle-like crystals of sodium urate. These contribute to the formation of kidney stones and produce the excruciating pain of gout when deposited in the joints.
Uric acid is only slightly soluble in water and easily precipitates out of solution forming needle-like crystals of sodium urate. These contribute to the formation of kidney stones and produce the excruciating pain of gout when deposited in the joints. The concentration of uric acid is 100-times greater in the cytosol than in the extracellular fluid. So when lethally-damaged cells release their contents, crystals of uric acid form in the vicinity.
So the risk of kidney stones and gout may be the price we pay for the antioxidant protection provided by uric acid.
Most mammals have an enzyme - uricas - for breaking uric acid down into a soluble product. However, during the evolution of great apes and humans, the gene encoding uricase became inactive. A predisposition to gout is our legacy.
- Kevin Ahern & Indira Rajagopal. Biochemistry Free & Easy. LibreTexts content adapted under CC BY-NC-SA 3.0 license.
- John W. Kimball. Biology. LibreTexts content adopted under Creative Commons Attribution 3.0 Unported (CC BY 3.0) license and made possible by funding from The Saylor Foundation.
- Wikipedia contributors. (2021, April 8). Uric acid. In Wikipedia, The Free Encyclopedia. Retrieved 20:30, April 10, 2021, from https://en.wikipedia.org/w/index.php?title=Uric_acid&oldid=1016645551