Purification and properties of murine corneal alcohol dehydrogenase. Evidence for class IV ADH properties

Human aldehyde dehydrogenase 3A1 (ALDH3A1): biochemical characterization and immunohistochemical localization in the cornea

ALDH3A1 (aldehyde dehydrogenase 3A1) is expressed at high concentrations in the mammalian cornea and it is believed that it protects this vital tissue and the rest of the eye against UV-light-induced damage. The precise biological function(s) and cellular distribution of ALDH3A1 in the corneal tissue remain to be elucidated. Among the hypotheses proposed for ALDH3A1 function in cornea is detoxification of aldehydes formed during UV-induced lipid peroxidation. To investigate in detail the biochemical properties and distribution of this protein in the human cornea, we expressed human ALDH3A1 in Sf9 insect cells using a baculovirus vector and raised monoclonal antibodies against ALDH3A1. Recombinant ALDH3A1 protein was purified to homogeneity with a single-step affinity chromatography method using 5'-AMP-Sepharose 4B. Human ALDH3A1 demonstrated high substrate specificity for medium-chain (6 carbons and more) saturated and unsaturated aldehydes, including 4-hydroxy-2-nonenal, which are generated by the peroxidation of cellular lipids. Short-chain aliphatic aldehydes, such as acetaldehyde, propionaldehyde and malondialdehyde, were found to be very poor substrates for human ALDH3A1. In addition, ALDH3A1 metabolized glyceraldehyde poorly and did not metabolize glucose 6-phosphate, 6-phosphoglucono-delta-lactone and 6-phosphogluconate at all, suggesting that this enzyme is not involved in either glycolysis or the pentose phosphate pathway. Immunohistochemistry in human corneas, using the monoclonal antibodies described herein, revealed ALDH3A1 expression in epithelial cells and stromal keratocytes, but not in endothelial cells. Overall, these cumulative findings support the metabolic function of ALDH3A1 as a part of a corneal cellular defence mechanism against oxidative damage caused by aldehydic products of lipid peroxidation. Both recombinant human ALDH3A1 and the highly specific monoclonal antibodies described in the present paper may prove to be useful in probing biological functions of this protein in ocular tissue.

Alcohol dehydrogenases and aldehyde dehydrogenases among inbred strains of mice: multiplicity, development, genetic studies and metabolic roles

Alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) are the major enzymes responsible for the metabolism of alcohols and aldehydes in the body. Both exist as a family of isozymes in mammals, and have been extensively studied in animal models, particularly among inbred strains of mice. Mouse ADH exists as at least three major classes, which are predominantly localized in liver (classes I and III), and in stomach/cornea (class IV). Mouse ALDH exhibits extensive multiplicity, several forms of which have been characterized, including ALDH1 (liver cytoplasmic/class 1 isozyme); ALDH2 (liver mitochondrial/class 2.); ALDH3 (stomach cytosolic/class 3); ALDH4 (liver microsomal/class 3); and ALDH5 (testis cytosolic/class 3). Biochemical, genetic and molecular genetic analyses have been performed on several of these enzymes, including studies on variant forms of ADH and ALDH. Distinct metabolic roles are proposed, based upon their tissue and subcellular distribution characteristics and the biochemical properties for these enzymes.

Molecular basis for differential substrate specificity in class IV alcohol dehydrogenases: a conserved function in retinoid metabolism but not in ethanol oxidation

Mammalian class IV alcohol dehydrogenase enzymes are characteristic of epithelial tissues, exhibit moderate to high K(m) values for ethanol, and are very active in retinol oxidation. The human enzyme shows a K(m) value for ethanol which is 2 orders of magnitude lower than that of rat class IV. The uniquely significant difference in the substrate-binding pocket between the two enzymes appears to be at position 294, Val in the human enzyme and Ala in the rat enzyme. Moreover, a deletion at position 117 (Gly in class I) has been pointed out as probably responsible for class IV specificity toward retinoids. With the aim of establishing the role of these residues, we have studied the kinetics of the recombinant human and rat wild-type enzymes, the human G117ins and V294A mutants, and the rat A294V mutant toward aliphatic alcohols and retinoids. 9-cis-Retinol was the best retinoid substrate for both human and rat class IV, strongly supporting a role of class IV in the generation of 9-cis-retinoic acid. In contrast, 13-cis retinoids were not substrates. The G117ins mutant showed a decreased catalytic efficiency toward retinoids and toward three-carbon and longer primary aliphatic alcohols, a behavior that resembles that of the human class I enzyme, which has Gly(117). The K(m) values for ethanol dramatically changed in the 294 mutants, where the human V294A mutant showed a 280-fold increase, and the rat A294V mutant a 50-fold decrease, compared with those of the respective wild-type enzymes. This demonstrates that the Val/Ala exchange at position 294 is mostly responsible for the kinetic differences with ethanol between the human and rat class IV. In contrast, the kinetics toward retinoids was only slightly affected by the mutations at position 294, compatible with a more conserved function of mammalian class IV alcohol dehydrogenase in retinoid metabolism.