The Major Transport Protein of Vitamin A: Retinol-Binding Protein (RBP)

December 23, 2025 •

2 min read

According to the World Health Organization, millions of children suffer from vitamin A deficiency globally, highlighting the importance of its efficient delivery throughout the body. The crucial protein responsible for the systemic transport of this vital nutrient is the retinol-binding protein (RBP), ensuring that vitamin A reaches the tissues that need it most.

Quick Summary

The retinol-binding protein (RBP) is the principal carrier of vitamin A in the bloodstream, transporting it from the liver to target tissues. It forms a complex with transthyretin (TTR) to prevent renal filtration. At the cellular level, the membrane receptor STRA6 mediates the uptake of vitamin A from RBP into cells.

Key Points

Retinol-Binding Protein (RBP): The major protein that specifically transports retinol, the active form of vitamin A, through the bloodstream.
RBP and Transthyretin (TTR): In plasma, RBP forms a complex with TTR, which prevents RBP from being lost through renal filtration.
STRA6 Receptor: A cell-surface receptor that facilitates the transport of retinol from the RBP complex into target cells.
Homeostasis and Toxicity: The RBP-mediated system provides a homeostatically controlled and safe delivery system, contrasting with the unregulated lipoprotein pathway that can lead to toxicity.
Liver Storage: The liver is the main storage site for vitamin A, and RBP is essential for mobilizing this reserve when needed by the body.
Targeted Delivery: The specific RBP-STRA6 interaction ensures that vitamin A is delivered precisely to the cells and tissues that require it.

Understanding the Vitamin A Transport Pathway

Vitamin A, specifically in the form of retinol, is a fat-soluble vitamin essential for vision, immune function, and embryonic development. Its hydrophobic nature means it cannot travel freely through the bloodstream and requires a specialized transport system. The major transport protein of vitamin A is retinol-binding protein (RBP), primarily synthesized and secreted by the liver.

The Liver's Central Role in RBP-Mediated Transport

After dietary absorption, vitamin A is stored in the liver as retinyl esters. When needed, these are converted to retinol and bound to RBP within liver cells, forming holo-RBP. This complex is then secreted into the bloodstream.

The RBP-Transthyretin Complex for Safe Passage

In the bloodstream, the RBP-retinol complex associates with transthyretin (TTR). This partnership is important as it prevents RBP from being filtered by the kidneys and excreted. The TTR binding also stabilizes RBP and helps ensure retinol is released only at target sites.

Cellular Uptake via the STRA6 Receptor

At target tissues, the RBP-TTR complex interacts with the cell-surface receptor STRA6. STRA6 facilitates the transfer of retinol from RBP directly into the cell. This process is distinct from internalizing the entire complex and is often aided by intracellular proteins like CRBP.

Comparison Table: RBP, TTR, and STRA6


Feature	Retinol-Binding Protein (RBP)	Transthyretin (TTR)	STRA6 Receptor
Primary Function	Specific carrier protein for retinol in the blood.	Forms complex with RBP to prevent renal filtration; also transports thyroxine.	Cellular membrane receptor for RBP that mediates vitamin A uptake.
Source/Origin	Primarily synthesized and secreted by the liver.	Synthesized mainly in the liver and choroid plexus.	A multi-transmembrane protein expressed in many organs.
Binding Partner	Binds retinol (vitamin A) and TTR.	Binds to the RBP-retinol complex.	Binds to the RBP-TTR complex on the cell surface.
Molecular Weight	~21 kDa.	~55 kDa (tetramer).	Integral membrane protein, larger than RBP.
Fate after Delivery	Returns to circulation as apo-RBP; cleared by kidneys.	Remains in circulation after releasing RBP.	Remains on the cell surface, facilitates uptake.

A Broader Perspective on Vitamin A Transport

While RBP is the main transport system for vitamin A, particularly under normal conditions, lipoproteins can also carry retinyl esters during high vitamin A intake. However, the lipoprotein pathway is less regulated and can increase the risk of vitamin A toxicity. The RBP-mediated pathway is crucial for controlled and safe delivery, ensuring tissues receive appropriate levels without excess.

Conclusion

In conclusion, the major protein responsible for transporting vitamin A (retinol) in the bloodstream is retinol-binding protein (RBP). Working in concert with transthyretin (TTR) for stability and to prevent renal loss, and utilizing the cell-surface receptor STRA6 for targeted delivery into cells, this complex system is vital for maintaining vitamin A balance and supporting essential bodily functions. Disruptions in this pathway can have significant health consequences.

Additional Resources

For a comprehensive review of the vitamin A transport mechanism mediated by the RBP receptor and STRA6, see: Vitamin A Transport Mechanism of the Multitransmembrane Cell-Surface Receptor STRA6.

Frequently Asked Questions

The primary protein for vitamin A transport in the blood is retinol-binding protein, or RBP.

RBP is a small protein, but it is prevented from being lost through kidney filtration by forming a larger complex with another protein called transthyretin (TTR).

When the RBP-TTR complex reaches a target cell, it binds to the cell-surface receptor STRA6, which facilitates the transfer of retinol into the cell.

No, the entire RBP-TTR complex is not internalized. The STRA6 receptor on the cell surface mediates the transfer of the retinol molecule, leaving the carrier proteins outside.

Yes, during excessive vitamin A intake, lipoproteins can also transport retinyl esters. However, this method is less regulated and can be associated with toxicity.

The liver is the central hub for vitamin A metabolism. It stores vitamin A as retinyl esters and synthesizes RBP to mobilize and transport retinol when the body needs it.

After retinol is transported across the cell membrane by STRA6, intracellular proteins like CRBPs bind to it. This helps maintain the concentration gradient and prevents the accumulation of free, toxic retinol inside the cell.