Vitamin A (retinol) and its natural derivatives are required for many physiological processes. The activity of retinoids is thought to be mediated by interactions with two subfamilies of nuclear retinoic acid receptors, RAR and RXR. The RARs bind all-trans retinoic acid (t-RA) with high affinity and alter gene expression as a consequence of this direct ligand interaction. RXR alpha is activated by t-RA, yet has little binding affinity for this ligand. t-RA may be converted to a more proximate ligand that directly binds and activates RXR alpha, and we have developed a method of nuclear receptor-dependent ligand trapping to test this hypothesis. Here we report the identification of a stereoisomer of retinoic acid, 9-cis retinoic acid, which directly binds and activates RXR alpha. These results suggest a new role for isomerization in the physiology of natural retinoids.

We measured all fins of 600 hatchery trout sampled from all 10 state fish hatcheries in Utah, and of wild fish sampled as controls comprising 58 rainbow trout (Oncorhynchus mykiss), 33 cutthroat trout (O. clarki), and 54 brown trout (Salmo trutta). A strong linear correlation was found between fin length and total body length (100–300 mm) for all fins of wild rainbow trout. "Relative fin length" (fin length/total body length × 100) proved to be a useful comparative measure, as this statistic was not biased by fish length in the wild fish sampled (all slopes <0.01%). Interspecific comparison of wild rainbow, cutthroat, and brown trout showed slight but statistically significant differences in some fin lengths. In intraspecific comparisons, hatchery fish had significantly shorter (10–50%) rayed fins than wild fish. The dorsal fin was most severely eroded in rainbow and brown trout, followed by the pectoral, anal, ventral, and caudal fins, in cutthroat trout the pattern was the same except that pectoral fins had more extensive erosion than dorsal fins. No species was clearly more susceptible to fin erosion in hatcheries, but the Fish Lake – DeSmet strain of rainbow trout had significantly shorter fins than other rainbow trout strains.

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