General Comments Biochemical Processes Involved

Vitamin A (retinol) mobilization from the liver is highly regulated by factors that control the rates of RBP synthesis and secretion by the liver. Considerable information is now available about the molecular and cellular events involved in the synthesis and secretion of RBP by the liver cell and about the mechanisms that regulate these events. The studies directed toward defining these phenomena have employed the rat as an animal model and a sensitive and specific radioimmunoassay for rat RBP (Muto et al., 1972). Studies have been conducted both with intact animals and with isolated liver cells in culture. 

A number of biochemical processes are involved in the phenomena of RBP synthesis and secretion. These include (1) biosynthesis of RBP (2) processing of the newly synthesized RBP molecule (3) translocation (directed movement) of the newly synthesized RBP molecule in the cell (4) complex formation of apo-RBP with retinol and (5) secretion of the retinol-RBP complex (holo-RBP) from the cell. The information currently available about these processes is reviewed below. 

One factor that exerts control over RBP secretion from the liver is the nutritional vitamin A (retinol) status of the animal. It is now well established that retinol deficiency specifically blocks the secretion of RBP from the liver so that 

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