Retinoic acid-binding protein (RABP) has been detected in the nuclei of chick embryo skin and Lewis lung tumor. The nuclear binding component showed the same ligand specificity and sedimentation value as the cytosol RABP. Whereas pronase completely digested the nuclear binding component, DNase showed 40%, and RNase showed negligible digestive action. Retinoic acid binding to the nuclear RABP was completely inhibited by a mercurial, and the inhibition was reversed by dithiothreitol. The nonspecific uptake of retinoic acid by Lewis lung nuclei and chick embryo skin nuclei was inhibited up to 50% by cytosol RABP. The maximal inhibitory effect produced by cytosol RABP was after 45-min incubation. Incubation of Lewis lung tumor with [3H]retinoic acid resulted in the appearance of nuclear RABP: [3H]retinoic acid in the nuclei. The complex formed was weak, and most of the bound retinoic acid could be removed by dialysis.

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This work was supported by Grant CA 18263 and Contract NO1-CP-22064 from the National Cancer Institute, Department of Health, Education and Welfare.

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