Intracellular binding proteins for retinol and retinoic acid: comparison with each other and with serum retinol-binding protein.

Soluble proteins with binding specificity for either retinol or retinoic acid were isolated from rat testis cytosol. The characteristics of the purified cytosolic protein were compared both with each other, and directly with those of serum retinol-binding protein (RBP). Substantial similarities were observed between the two cytosol retinoid binding proteins, as well as clear differences between each of these proteins and serum RBP. The intracellular binding proteins differed from serum RBP with regard to molecular weight, immuno-reactivity, binding affinity for serum prealbumin, and ultraviolet and fluorescence spectral characteristics. The ultraviolet absorption spectrums of the cytosol binding protein for retinol (CRBP) and of the cytosol binding protein for retinoic acid (CRABP) were almost identical despite differences in the nature of the ligands bound by these two proteins.