This is the "parent" vitamin A molecule from which tRA is derived. All-trans-retinol is first converted to retinal and then tRA by two dehydrogenase enzymes, presumably specific. It is found at high levels endogenously in mammalian and chick embryos (3-5). Usually it is about 10-fold less potent than tRA in most biological assays (6), including mammalian teratogenicity (7), but it is inactive in inducing pattern duplication when applied locally to the regenerating amphibian limb (8), the chick limb bud (9), or developing chick skin (10).

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