Isomer-specific retinoic acid biosynthesis in HeLa cells expressing recombinant class I aldehyde dehydrogenases

Retinal dehydrogenase type 1 (RALDH1) catalyzes the oxidation of all-trans and 9-cis retinal to the respective retinoic acids (RAs), whereas another member of the aldehyde dehydrogenase family, the phenobarbital-induced aldehyde dehydrogenase (PB-ALDH), is very poorly active. We have previously generated chimeras between these two enzymes that displayed selectivity for retinal isomers in crude bacterial extracts. To examine whether the selectivity of the recombinant enzymes is retained in intact cells, we first assessed whether retinoid-isomerizing activity is present in cultured eukaryotic cells. Our results demonstrate that the only RA isomers detected in RALDH1-expressing or non-expressing cells corresponded to the same steric conformation as the supplied retinoids, indicating a lack of measurable 9-cis/all-trans retinoid-isomerizing activity. Finally, HeLa cells transfected with RALDH1 derivatives that were retinal isomer-selective in vitro produced only the corresponding RA isomers, establishing these enzymes as useful tools to assess the respective roles of the two RA isomers in vivo.