Which binding protein modulates the bioavailability of IGF-I?

Which binding protein modulates the bioavailability of IGF-I?

• A. IGF-I
• B. IGFBP-III
• C. Albumin
• D. Transferrin

Answer: (B)

IGF-I’s activity in the blood is governed by how much is free versus bound to binding proteins. The protein that most strongly modulates IGF-I bioavailability is IGF binding protein-3. It binds IGF-I in circulation and forms a ternary complex with an acid-labile subunit, which greatly extends IGF-I’s half-life and limits immediate receptor activation. This binding acts as a reservoir, controlling how much IGF-I is readily available to interact with IGF-1 receptors when needed. Because of this, the circulating pool of IGF-I is largely complexed with IGFBP-3, rather than freely floating. Other proteins like albumin serve as general carriers for many substances but do not specifically regulate IGF-I bioavailability in the same targeted way. Transferrin is involved in iron transport, not IGF-I regulation. IGF-I itself is the hormone, and binding proteins are what tune how much of it can act at receptors.

IGF-I’s activity in the blood is governed by how much is free versus bound to binding proteins. The protein that most strongly modulates IGF-I bioavailability is IGF binding protein-3. It binds IGF-I in circulation and forms a ternary complex with an acid-labile subunit, which greatly extends IGF-I’s half-life and limits immediate receptor activation. This binding acts as a reservoir, controlling how much IGF-I is readily available to interact with IGF-1 receptors when needed.

Because of this, the circulating pool of IGF-I is largely complexed with IGFBP-3, rather than freely floating. Other proteins like albumin serve as general carriers for many substances but do not specifically regulate IGF-I bioavailability in the same targeted way. Transferrin is involved in iron transport, not IGF-I regulation. IGF-I itself is the hormone, and binding proteins are what tune how much of it can act at receptors.