Leptin is a 16 kDa protein hormone that plays a key role in regulating energy intake and energy expenditure, including appetite and metabolism. It is one of the most important adipose derived hormones. The ''Ob(Lep)'' gene (Ob for obese, Lep for leptin) is located on chromosome 7 in humans.
The effects of leptin were observed by studying mutant obese mice that arose at random within a mouse colony at the Jackson Laboratory in 1950. These mice were massively obese and excessively voracious. Ultimately, several strains of laboratory mice have been found to be homozygous for single-gene mutations that causes them to become grossly obese, and they fall into two classes: "ob/ob", those having mutations in the gene for the protein hormone leptin, and "db/db", those having mutations in the gene that encodes the receptor for leptin. When ob/ob mice are treated with injections of leptin they lose their excess fat and return to normal body weight.
Leptin itself was discovered in 1994 by Jeffrey M. Friedman and colleagues at the Rockefeller University through the study of such mice.
Human leptin is a protein of 167 amino acids. It is manufactured primarily in the adipocytes of white adipose tissue, and the level of circulating leptin is directly proportional to the total amount of fat in the body.
In addition to white adipose tissue—the major source of leptin—it can also be produced by brown adipose tissue, placenta (syncytiotrophoblasts), ovaries, skeletal muscle, stomach (lower part of fundic glands), mammary epithelial cells, bone marrow, pituitary and liver.
Leptin has also been discovered to be synthesised from Gastric Chief Cells and P cells in the stomach.
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