Partial characterization of molecular species of retinol-binding protein found in tubular proteinuria due to chronic cadmium poisoning in the rabbit.

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Abstract
A study was conducted to characterize the molecular species of retinol-binding protein (RBP) isolated from the urine of rabbits chronically poisoned with Cd. The RBP species, identical with regard to both molecular size (approximately 20,000) and immunoreactivity, were separated into 4 fractions by means of polyacrylamide gel electrophoresis (PAGE), which yielded 2 holo-RBP (H2 and H1) and 2 apo-RBP (A2 and A1) species. The urinary excretion ratio of these fractions (H2: H1: A2: A1) was about 70: 6: 21: 3. No distinct difference of amino acid composition between holo- and apo-RBP was observed. An additional species of apo-RBP (designated An) was also isolated from the aged holo-RBP (H2) by isoelectric focusing in gel. Using the separated molecular species of rabbit RBP and of human RBP, their interactions with human prealbumin (PA) were examined both by human PA-Sepharose affinity chromatography, and by gel filtration on Sephadex G-100 after in vitro incubation of the RBP with human PA. Purified rabbit holo-RBP exhibited almost the same binding ability to human PA as did human RBP. Retinol within RBP molecule enhanced the affinity to PA, presumably through the change of tertiary structure, although the presence of retinol was not essential for the protein-protein interaction.

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