β-Oxidation in rabbit liver in vitro and in the perfused ferret liver contributes to retinoic acid biosynthesis from β-apocarotenoic acids

Abstract

The biosynthesis of retinoic acid from β-apocarotenoic acids was examined for a β-oxidation-like process using both rabbit liver mitochondrial fractions with various β-apocarotenoic acids (β-apo-14'-, β- apo-12'-, and β-apo-8'-carotenoic acid) and perfusion in ferret liver through the portal vein with β-apo-8'-carotenoic acid. The in vitro incubation of β-apo-8', β-apo-12'-, and β-apo-14'-carotenoic acids gave rise to shorter chain β-apo-carotenoic acids as well as retinoic acid. The rate of retinoic acid synthesis from 10 pμ β-apo-8', β-apo-12'-, and β- apo-14'-carotenoic acids was 11 ± 2, 18 ± 3, and 30 ± 7 pmol/h/mg of protein, respectively. The stepwise oxidation of β-apocarotenoic acid in mitochondria was dose-related to both protein concentration and substrate concentration. β-Apocarotenoic acid oxidation was inhibited in a dose- dependent manner when it was co-incubated with oleoyl-CoA. The in vivo perfusion of ferret liver with β-apo-8'-carotenoic acid resulted in a linear increase in the retinoic acid concentration of bile, which was completely abolished by co-perfusion of 3-mercaptopropionic acid, an inhibitor of long chain acyl-CoA dehydrogenase, and partially inhibited by 2- tetradecylglycidic acid, an inhibitor of carnitine-palmitoyl-CoA transferase I. However, the formation of retinoic acid from the β-apocarotenoic acids was not inhibited, either in vitro or in vivo, by citral, an inhibitor of retinal oxidase. Thus, the formation of retinoic acid was not occurring by the central cleavage pathway. These data suggest that the oxidation of intermediate compounds between β-carotene and retinoic acid may undergo a type of β-oxidative process to form retinoic acid, which is reminiscent of mitochondrial fatty acid β-oxidation. This pathway may play an important role in the biosynthesis of retinoic acid from β-carotene.