Analysis of the NADH-dependent retinaldehyde reductase activity of amphioxus retinol dehydrogenase enzymes enhances our understanding of the evolution of the retinol dehydrogenase family

In vertebrates, multiple microsomal retinol dehydrogenases are involved in reversible retinol/retinal interconversion, thereby controlling retinoid metabolism and retinoic acid availability. The physiologic functions of these enzymes are not, however, fully understood, as each vertebrate form has several, usually overlapping, biochemical roles. Within this context, amphioxus, a group of chordates that are simpler, at both the functional and genomic levels, than vertebrates, provides a suitable evolutionary model for comparative studies of retinol dehydrogenase enzymes. In a previous study, we identified two amphioxus enzymes, Branchiostoma floridae retinol dehydrogenase 1 and retinol dehydrogenase 2, both candidates to be the cephalochordate orthologs of the vertebrate retinol dehydrogenase enzymes. We have now proceeded to characterize these amphioxus enzymes. Kinetic studies have revealed that retinol dehydrogenase 1 and retinol dehydrogenase 2 are microsomal proteins that catalyze the reduction of all-trans-retinaldehyde using NADH as cofactor, a remarkable combination of substrate and cofactor preferences. Moreover, evolutionary analysis, including the amphioxus sequences, indicates that Rdh genes were extensively duplicated after cephalochordate divergence, leading to the gene cluster organization found in several mammalian species. Overall, our data provide an evolutionary reference with which to better understand the origin, activity and evolution of retinol dehydrogenase enzymes.