9-cis Retinoic acid is a highly preferred ligand for the nuclear retinoic acid receptor RXR. Although 9-cis retinoic acid can be formed from all-trans retinoic acid in vivo and in vitro, the nature of the isomerization process has not been defined. 9-cis-Retinoic acid might also be formed biologically from 9-cis β-carotene, an isomer of β-carotene commonly found in foods. We have observed that enzyme preparations of rat liver and intestine convert highly purified 9-cis β-carotene (99.8%) to a mixture of 9-cis, all-trans, and 13-cis retinals. Highly purified all-trans β-carotene (99.7%) and 13-cis β-carotene (99%) are primarily converted to all-trans retinal. Several aldehyde dehydrogenases in tissues can convert retinal to retinoic acid. Although the rates of cleavage of 9-cis and 13-cis β-carotenes are slower than that of the all-trans isomer, the oxidative conversion of 9-cis β-carotene to 9-cis and all-trans retinals is the first identified enzymatic reaction for the formation of 9-cis retinal in animal tissues.— Nagao, A., Olson, J. A. Enzymatic formation of 9-cis, 13-cis, and all-trans retinals from isomers of β-carotene. FASEB J. 8: 968-973; 1994.