Effects of dietary deficiencies of protein and retinol on the plasma level of retinol-binding protein in the rat.

I . The plasma concentration of retinol-binding holoprotein (holo-RBP) in rats given retinol-sufficient diets containing 50 g soya-bean protein/kg was 90 yo of the concentration found in controls given zoo g soya-bean protein/kg, whereas in rats given 50 g rice protein/kg it was only 68 % of the control value. Plasma albumin concentrations in the low-protein groups were lower by 20-25 %. 2. The rats given retinol-deficient diets, whose plasma levels of holo-RBP were reduced almost to zero after 5-8 weeks, were given 1.5 mg retinol/kg body-weight. Changes in plasma holo-RBP were measured for 7 d. The rapid rise in the latter to a peak above normal control levels within I .5-z h indicated that a pool of the apoprotein had accumulated in the liver during the period of deficiency and that retinol was necessary for its release. 3. The maximum value reached in the groups of rats given the rice-protein diet was smaller than the values for the groups given the soya-bean-protein diets, reflecting a reduced rate of synthesis of the carrier protein in the livers of those groups. 4. The administration of IOO g casein/kg diet to the rats receiving only 50 g rice-protein/kg immediately stimulated the synthesis of the carrier protein which was released into plasma, reaching a peak after 24 h before settling down after 3-4 d to the steady-state level expected for normal well-nourished controls. 5. The half-life for excess holo-RBP released into plasma of rats receiving a normal level of good-quality protein was 7 h, where it was 14 h for the groups of low-protein diets.