Ammonia, derived from protein and nucleic acid catabo-lism, plays a pivotal role in nitrogen metabolism and is needed in the biosynthesis of nonessential amino acids and nucleic acids. Ammonia metabolism is a major function of the liver. The liver has an ammonia level 10 times higher than the plasma ammonia level. High circulating ammonia levels are highly neurotoxic, and a deficiency in hepatic function can lead to several distinct neurological disorders, including coma in severe cases.
The liver synthesizes most of the urea in the body. The enzymes involved in the urea cycle are regulated by protein intake. In humans, starvation stimulates these enzymes.
Cholesterol Metabolism. The liver plays an important role in cholesterol homeostasis. Liver cholesterol is derived from both de novo synthesis and the lipoproteins taken up by the liver. Hepatic cholesterol can be used in the formation of bile acids, biliary cholesterol secretion, the synthesis of VLDLs, and the synthesis of liver membranes. Because the absorption of biliary cholesterol and bile acids by the GI tract is incomplete, this method of eliminating cholesterol from the body is essential and efficient. However, patients with high plasma cholesterol levels might be given additional drugs, such as statins, to lower their plasma cholesterol levels. Statins act by inhibiting enzymes that play an essential role in cholesterol synthesis. VLDLs secreted by the liver provide cholesterol to organs that need it for the synthesis of steroid hormones (e.g., the adrenal glands, ovaries, and testes).
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