Metabolism of the Vitamin A Transporting Protein Complex

Abstract. Biological turnover studies of the human vitamin A transporting protein complex and its individual components, prealbumin and retinol binding protein (RBP) have been performed in normal persons and in patients with severely impaired kidney function. The metabolic behaviour of prealbumin was very similar in the two groups of subjects studied, and it was concluded that prealbumin eatabolism can be dependent to only a minor extent on normally functioning kidney glomeruli.–RBP is present in several molecular forms in plasma. In normal conditions virtually all PRB is bound to prealbumin, but small amounts of RBP also occur in free form. The concentration of the free RBP is greatly increased in patients with grossly impaired glomerular filtration. The biological half‐life for free RBP was short in normal persons (about 4 hours) whereas in the patients this parameter was increased 10 to 15 fold. These findings suggest that most if not all free RBP in plasma is catabolized by the kidney. RBP complexed with prealbumin has a considerably longer half‐life than its free counterpart in normal persons but the absolute synthetic rate of the two components of RBP is similar. Thus it is proposed that most RBP is catabolized from the pool of the uneomplexed species.–From the data it can be calculated that as much as one third of the plasma vitamin A may be deposited in the kidney tubuli on degradation of RBP, since the mechanism for the renal eatabolism of this small protein seems to involve glomerular filtration followed by tubular reabsorption.