A retinoic acid-binding protein from chick embryo skin

Previous studies by others have indicated that retinoic acid reverses metaplastic changes caused by chemical caricinogens in vivo and in organ culture. The present study deals with the detection of a specific retinoic acid-binding protein from chick embryo metatarsal skin that may be mediating these biological effects. The protein has an S20 value of 2 and an isoelectric pH of 4.6. Competition experiments with labeled retinoic acid and 200-fold molar excesses of unlabeled retinoic acid, retinol, retinal, methyl retinoate, diethylretinamide, synthetic analogs of retinoic acid, and gamma-linolenic acid reveal that only retinoic acid and its analogs with a free carboxyl group bind to this protein. Among the analogs of retinoic acid, a cyclopentenyl analog, a trimethylmethoxyphenyl analog, 13-cis-retinoic acid, and alpha-retinoic acid compete for the binding site on the protein, with the cyclopentenyl analog having greater affinity than retinoic acid does. Phenyl and pyridyl analogs of retinoic acid are poor binders. In general, the ability of the various analgos to bind to this protein correlates with their biological activity in the reversal of keratinization and in the production of mucous metaplasia by chick embryo metatarsal skin as reported by others.