Molecular cloning and baculovirus expression of the rabbit corneal aldehyde dehydrogenase (ALDH1A1) cDNA

Most mammalian species express high concentrations of ALDH3A1 in corneal epithelium with the exception of the rabbit, which expresses high amounts of ALDH1A1 rather than ALDH3A1. Several hypotheses that involve catalytic and/or structural functions have been postulated regarding the role of these corneal ALDHs. The aim of the present study was to characterize the biochemical properties of the rabbit ALDH1A1. We have cloned and sequenced the rabbit ALDH1A1 cDNA, which is 2,073 bp in length (excluding the poly(A+) tail), and has 5' and 3' nontranslated regions of 46 and 536 bp, respectively. This ALDH1A1 cDNA encodes a protein of 496 amino acids (Mr = 54,340) that is: 86-91% identical to mammalian ALDH1A1 proteins, 83-85% identical to phenobarbital-inducible mouse and rat ALDH1A7 proteins, 84% identical to elephant shrew ALDH1A8 proteins (eta-crystallins), 69-73% identical to vertebrate ALDH1A2 and ALDH1A3 proteins, 65% identical to scallop ALDH1A9 protein (omega-crystallin), and 55-57% to cephalopod ALDH1C1 and ALDH1C2 (omega-crystallins). Recombinant rabbit ALDH1A1 protein was expressed using the baculovirus system and purified to homogeneity with affinity chromatography. We found that rabbit ALDH1A1 is catalytically active and efficiently oxidizes hexanal (Km = 3.5 microM), 4-hydroxynonenal (Km = 2.1 microM) and malondialdehyde (Km = 14.0 microM), which are among the major products of lipid peroxidation. Similar kinetic constants were observed with the human recombinant ALDH1A1 protein, which was expressed and purified using similar experimental conditions. These data suggest that ALDH1A1 may contribute to corneal cellular defense against oxidative damage by metabolizing toxic aldehydes produced during UV-induced lipid peroxidation.