Abstract
Retinol circulates in rat plasma, in the form of being bound to a specific transport protein: Retinol Binding Protein (RBP). A sensitive, specific radioimmunoassay for rat RBP was developed. By means of this assay, a study was conducted to examine the effects of vitamin A (VA) depletion and deficiency, and of repletion, on the level of serum RBP, in order to explore the role of nutritional VA status in the regulation of RBP metabolism in the rat. In VA deficient rats, the serum VA levels decreased markedly (almost zero) and the serum RBP levels also declined (15 μg/ml) with a time-course similar to that observed for VA, but with a lag of about 3 days: most of the circulating RBP was present as apo-RBP. However, in vivo supplementation with retinoic acid in VA deficient rats did not recover the serum RBP to the normal level.
The level of immunoreactive RBP in livers of deficient rats was found to be 4 times higher than that in control rat livers (p<0.001). Furthermore, in vivo administration of VA to deficient rats immediataly restored the low serum RBP level toward the normal (within 5 hours).
These findings strongly suggest that VA deficiency primarily interferes with secretion, rather than with synthesis, of RBP by the liver, and that the deficient liver contains a pool of preformed apo-RBP which can be released rapidly into serum, as holo-RBP, when VA becomes available
