The complex between retinol and retinol-binding protein is formed in the rough microsomes of liver following repletion of vitamin A-depleted rats.

Retinol-binding protein (RBP), the plasma transport protein for vitamin A, is primarily synthesized in the rough endoplasmic reticulum of the liver. RBP then passes through the smooth endoplasmic reticulum and into the Golgi apparatus where vesicles form and transport the protein to the cell membrane. When rats were depleted of their vitamin A stores, RBP accumulated in the liver microsomes, particularly in the rough microsomes. To identify the organelle(s) where retinol initially binds to RBP, vitamin A-depleted rats were given an i.v. injection of [3H]retinol suspended in Tween 40. After intervals of 2, 3, 4, 5, 6, 8, 10, 15 and 20 min, liver fractions enriched in rough and smooth microsomes and Golgi apparatus were prepared. The retinol/RBP complex (holoRBP) was detected in the rough microsomes within 3 min post injection. HoloRBP later appeared in the smooth microsomes and Golgi fraction, and then the serum at time intervals consistent with the known secretion rate for RBP. HoloRBP was detected in the rough microsomes at all times after 3 min, whether or not the complex was present in the other subcellular fractions. Thus, the holoRBP complex can form in the rough endoplasmic reticulum of the liver.