The application of retinoic acid (RA) to the developing chick limb bud causes 6-digit double posterior limbs to form instead of the normal 3-digit limb. As an attempt to begin a molecular analysis of this phenomenon we have identified and characterized a soluble cytoplasmic receptor for RA, namely cytoplasmic retinoic acid-binding protein (CRABP), from the cells of the chick limb bud. It is present from stages 20–35 at similar levels and has an apparent Kd of 140–280 nM. In competition experiments with other retinoids Ro 13-7410 was found to be the most effective at competing for sites on CRABP followed by all-trans-RA, 13-cis-RA, Ro 10-1670 and retinal. Retinol, retinyl palmitate, retinyl acetate, etretinate and arotinoid showed low or no affinity for CRABP. Specificity for binding was thus demonstrated since analogues with an acid end group competed effectively, the aldehyde competed less effectively and the ester or alcohol groups did not compete. At the concentration of RA that needs to be administered to cause duplications in the pattern of the limb bud, we estimate that 4% of the CRABP present in the limb bud has RA bound. The similarities between steroid receptors in the mediation of steroid hormone action and CRABP in the mediation of RA action is discussed. In this regard we note that while there are 104 steroid receptors per cell in other cell types we estimate that there are about 105 RA receptors per cell in the chick limb bud.

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