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Ethanol is teratogenic to many vertebrates. We are utilizing zebrafish as a model system to determine whether there is an association between ethanol metabolism and ethanol-mediated developmental toxicity. Here we report the isolation and characterization of two cDNAs encoding zebrafish alcohol dehydrogenases (ADHs). Phylogenetic analysis of these zebrafish ADHs indicates that they share a common ancestor with mammalian class I, II, IV, and V ADHs. The genes encoding these zebrafish ADHs have been named Adh8a and Adh8b by the nomenclature committee. Both genes were genetically mapped to chromosome 13. The 1450-bp Adh8a is 82, 73, 72, and 72% similar at the amino acid level to the Baltic cod ADH8 (previously named ADH1), the human ADH1B2, the mouse ADH1, and the rat ADH1, respectively. Also, the 1484-bp Adh8b is 77, 68, 67, and 66% similar at the amino acid level to the Baltic cod ADH8, the human ADH1B2, the mouse ADH1, and the rat ADH1, respectively. ADH8A and ADH8B share 86% amino acid similarity. To characterize the functional properties of ADH8A and ADH8B, recombinant proteins were purified from SF-9 insect cells. Kinetic studies demonstrate that ADH8A metabolizes ethanol, with a V max of 13.4 nmol/min/mg protein, whereas ADH8B does not metabolize ethanol. The ADH8A K m for ethanol as a substrate is 0.7 mM. 4-Methyl pyrazole, a classical competitive inhibitor of class I ADH, failed to inhibit ADH8A. ADH8B has the capacity to efficiently biotransform longer chain primary alcohols (>5 carbons) and S-hydroxymethlyglutathione, whereas ADH8A does not efficiently metabolize these substrates. Finally, mRNA expression studies indicate that both ADH8A and ADH8B mRNA are expressed during early development and in the adult brain, fin, gill, heart, kidney, muscle, and liver. Together these results indicate that class Ilike ADH is conserved in zebrafish, albeit with mixed functional properties.Fetal alcohol syndrome in children born to women who consumed alcohol during pregnancy was first described by Jones and colleagues in 1973 (1). Fetal alcohol syndrome is characterized by a delay in development, cardiac abnormalities (1), central nervous abnormalities, abnormal craniofacial features, and intellectual delays (1, 2). The teratogenic properties of ethanol have been firmly established; however, the underlying mechanism(s) of toxicity remain unclear. Two molecular mechanisms have been proposed: direct ethanol effects and the indirect effects associated with acetaldehyde formation and oxidative stress (3). The ability of ethanol to cause developmental anomalies has been demonstrated in mice, rats, Drosophila melanogaster, and chickens (4 -7). Zebrafish are well suited for genetic studies (reviewed in Ref. 8). Zebrafish embryos exposed to ethanol display craniofacial abnormalities, cardiac and structural malformations, and development delay (9 -11). Visual function was affected in embryos exposed to 1.5% ethanol during development (12). Recent studies indicate that three adult strains of zebrafish display different behavior...

ABSTRACT:Ethanol is metabolized to acetaldehyde mainly by the alcohol dehydrogenase pathway and, to a lesser extent, through microsomal oxidation (CYP2E1) and the catalase-H 2 O 2 system. Acetaldehyde, which is responsible for some of the deleterious effects of ethanol, is further oxidized to acetic acid by aldehyde dehydrogenases (ALDHs), of which mitochondrial ALDH2 is the most efficient. The aim of this study was to evaluate zebrafish (Danio rerio) as a model for ethanol metabolism by cloning, expressing, and characterizing the zebrafish ALDH2. The zebrafish ALDH2 cDNA was cloned and found to be 1892 bp in length and encoding a protein of 516 amino acids (M r ‫؍‬ 56,562), approximately 75% identical to mammalian ALDH2 proteins. Recombinant zebrafish ALDH2 protein was expressed using the baculovirus expression system and purified to homogeneity by affinity chromatography. We found that zebrafish ALDH2 is catalytically active and efficiently oxidizes acetaldehyde (K m ‫؍‬ 11.5 M) and propionaldehyde (K m ‫؍‬ 6.1 M). Similar kinetic properties were observed with the recombinant human ALDH2 protein, which was expressed and purified using comparable experimental conditions. Western blot analysis revealed that ALDH2 is highly expressed in the heart, skeletal muscle, and brain with moderate expression in liver, eye, and swim bladder of the zebrafish. These results are the first reported on the cloning, expression, and characterization of a zebrafish ALDH, and indicate that zebrafish is a suitable model for studying ethanol metabolism and, therefore, toxicity.

It is largely accepted that vertebrates are more susceptible to chemical insult during the early life stage. It is implied that if a chemical such as ethanol is developmentally toxic, it must interfere with, or modulate, critical signaling pathways. The probable molecular explanation for increased embryonic susceptibility is that collectively there is no other period of an animal's lifespan when the full repertoire of molecular signaling is active. Understanding the mechanism by which ethanol exposure disrupts vertebrate embryonic development is enormously challenging; it requires a thorough understanding of the normal molecular program to understand how transient ethanol exposure disrupts signaling and results in detrimental long-lasting effects. During the past several years, investigators have recognized the advantages of the zebrafish model to discover the signaling events that choreograph embryonic development. External development coupled with the numerous molecular and genetic methods make this model a valuable tool to unravel the mechanisms by which ethanol disrupts embryonic development. In this chapter we describe procedures used to evaluate and define the morphological, cellular and molecular responses to ethanol in zebrafish.