Cloning of a cDNA Encoding an Aldehyde Dehydrogenase and Its Expression in

Abstract

The biosynthesis of the hormone retinoic acid from retinol (vitamin A) involves two sequential steps, cata-lyzed by retinol dehydrogenases and retinal dehydroge-nases, respectively. This report describes the cloning of a cDNA encoding a heretofore unknown aldehyde dehy-drogenase from a rat testis library and its expression in Escherichia coli. This enzyme has been designated reti-nal dehydrogenase, type II, RalDH(II). The deduced amino acid sequence of RalDH(II) had the highest iden-tity with mammalian aldehyde dehydrogenases that fea-ture low K m values (␮M) for retinal: human ALDH1 (72.2%), rat retinal dehydrogenase, type I (71.5%), bovine retina (72.7%), and mouse AHD-2 (71.5%). RalDH(II) ex-pressed in E. coli recognizes as substrates free retinal, with a K m of ϳ0.7 ␮M, and cellular retinol-binding pro-tein-bound retinal, with a K m of ϳ0.2 ␮M. RalDH(II) also can utilize as substrate retinal generated in situ by mi-crosomal retinol dehydrogenases, from the physiologi-cally most abundant substrate: retinol bound to cellular retinol-binding protein. Rat testis expresses RalDH(II) mRNA most abundantly, followed by (relative to testis): lung (6.7%), brain (6.3%), heart (5.2%), liver (4.4%), and kidney (2.7%). RalDH(II) does not recognize citral, benz-aldehyde, acetaldehyde, and propanal efficiently as sub-strates, but does metabolize octanal and decanal effi-ciently. These data support a function for RalDH(II) in the pathway of retinoic acid biogenesis.