The Urea Cycle

The urea cycle is a cycle of biochemical reactions that produces urea (NH2)2CO from ammonia (NH3). The urea cycle converts highly toxic ammonia to urea for excretion. This cycle was the first metabolic cycle to be discovered (Hans Krebs and Kurt Henseleit, 1932), five years before the discovery by Krebs of the TCA cycle (also known as Krebs cycle and citric acid cycle). The urea cycle takes place primarily in the liver and, to a lesser extent, in the kidneys. 尿素循环是由一系列生化反应组成的循环，它将氨（NH₃）转化为尿素（(NH₂)₂CO）。尿素循环将高度有毒的氨转化为尿素以便排出体外。这个循环是第一个被发现的代谢循环（1932年，汉斯·克雷布斯和库尔特·恩塞莱特发现），比克雷布斯发现三羧酸循环（也称为克雷布斯循环和柠檬酸循环）早五年。尿素循环主要在肝脏中进行，其次在肾脏中也有所发生。

The chemical process essentially combines two ammonia molecules with a carbon dioxide molecule to make a urea molecule. The cycle of reactions that produces a molecule of urea to send to the kidneys uses the molecule ornithine as a "carrier" to collect and deliver the ammonia as output in the form of urea. Most of the structure of ornithine is present in each of the intermediate steps and is reformed as ornithine to repeat the cycle after having released the urea molecule for its journey to the kidney and excretion.

The liver is the main location for the urea cycle, where part of the cycle and supporting reactions occur in the mitochondria of liver cells and the remainder in the cytosol of those cells. Following the construction of the urea, it is transported in the bloodstream to the kidneys where the urea is filtered out for excretion. The measured level of the blood urea nitrogen (BUN) is a sensitive test of kidney function since filtration and excretion are impaired if the kidneys are malfunctioning. Another blood test to determine the concentration of ammonia is a sensitive test of liver function since a normally functioning liver would keep the ammonia level low.

The Urea Cycle

The diagram of the urea cycle above is patterned after the treatment in Matthews, et al. to show the main chemical compounds involved in the preparation of urea for the purpose of disposal of toxic ammonia from the body. Note that the process requires energy in the form of 3 ATPs per cycle. It also makes use of NADH in an energy exchange. Some of the reactions take place in the mitochondria of liver cells, while the remainder take place in the cytosol of those cells.

Urea

Urea is the key compound excreted by the kidney to remove toxic ammonia from the body. Before being transported to the kidneys, the urea is produced in the urea cycle in the liver by conversion of the NH4+ from the body's metabolism. 尿素是肾脏排出体内的有毒氨的关键化合物。在肝脏中，尿素循环将体内代谢产生的NH₄⁺转化为尿素，在运输至肾脏之前产生。

Urea was synthesized from inorganic matter by Friedrich Wohler in 1828 and is reputed to be the first organic molecule or "molecule of life" ever synthesized. He is credited as being one of the fathers of biochemistry. 尿素于1828年由弗里德里希·沃勒尔从无机物合成，被认为是第一个有机分子或“生命分子” ever 合成的。他被公认为生物化学的奠基人之一。