Insulin-like Growth Factor I (IGF-I)
Insulin-like Growth Factor I (IGF-1) is produced by the liver and target tissues. Production is stimulated by growth hormone and retarded by undernutrition. A large fraction of circulating IGF-1 is complexed with IGF binding proteins. IGF-I is closely related to a second protein, IGF-II. IGF-II also binds the IGF-1 Receptor, and mediates growth in the same way that IGF-I does. However, IGF-2 alone binds the IGF II receptor (also called the Mannose-6 phosphate receptor), that appears to be a non-signaling receptor. IGF-I is produced throughout life with the highest rates of IGF-I production occurring during the pubertal growth spurt and the lowest levels occurring during infancy and old age. IGF-I levels can be measured in the blood in 10-1000 ng/ml amounts. As levels do not fluctuate greatly throughout the day, IGF-I is used by physicians as a screening test for growth hormone deficiency and excess. As the name "insulin-like growth factor I" implies, IGF-I is structurally related to insulin and is even capable of binding the insulin receptor, albeit at lower affinity than insulin.
Swiss-Prot Accession Number: P01343