Demonstration of a novel molecular species in chick embryo brain: cellular retinol-binding protein, F-type.

By sucrose density gradient sedimentation analysis, a cytosol component of small molecular size which bound 3H-retinoic acid was detected in the chick embryo brain, exhibiting a peak on day 14 (7 days before hatching). In general, cellular retinol-binding protein (CRBP) and cellular retinoic acid-binding protein (CRABP) have been distinguished by showing binding specificity for retinol or retinoic acid, respectively. However, the component found in embryo brain cytosol exhibited binding affinity for both retinol and retinoic acid, being similar in less ligand-specificity to CRBP (F) which was originally recognized in fish eye cytosol. Moreover, the component also showed a lack of binding affinity for prealbumin (PA), excluding the possibility that chicken plasma RBP was contaminated in the cytosol. These findings strongly suggest that a distinct molecular species of cellular binding protein for vitamin A exists in the brain cytosol of a developing chick embryo.