RELATIONSHIP OF HOLO-FREE AND TRANSTHYRETIN-BOUND PLASMA RETINOL-BINDING PROTEIN-LEVELS WITH LIVER VITAMIN-A IN RATS

Vitamin A containing (holo) free and transthyretin-bound (TTR) retinol binding protein (RBP) concentrations in plasma from rats with variable vitamin A status were determined by high performance liquid chromatography. Two different methods were used: (1) molecular exclusion with a TSK 2000 column and (2) reverse phase using a Protesil Octyl 300 column. Holo TTR-RBP peak areas were positively correlated to liver vitamin A concentrations (r = 0.79 for molecular exclusion, 0.81 for reverse phase) in rats with marginal and normal vitamin A status. This correlation was higher than the correlation of serum retinol to liver vitamin A in these rats (r = 0.58). The correlation of holo-TTR-RBP to liver vitamin A was also higher than its correlations to plasma vitamin A. Therefore, plasma concentrations of holo-TTR-RBP may be influenced by marginal vitamin A liver stores to a greater extent than plasma retinol is. This suggests that holo-TTR-RBP protein concentrations may be the better indicator of marginal vitamin A nutritional status in rats. The correlations of both holo-TTR-RBP and serum retinol decreased sharply at high liver vitamin A concentrations. Neither method is suitable for measuring sub-toxicity in rats.