Amphioxus alcohol dehydrogenase is a class 3 form of single type and of structural conservation but with unique developmental expression

The coding region of amphioxus alcohol dehydrogenase class 3 (ADH3) has bee n characterized from two species, Branchiostoma lanceolatum and Branchiosto ma floridae. The species variants have residue differences at positions tha t result in only marginal functional distinctions. Activity measurements sh ow a class 3 glutathione-dependent formaldehyde dehydrogenase, with k(cat)/ K-m values about threefold those of the human class 3 ADH enzyme. Only a si ngle ADH3 form is identified in each of the two amphioxus species, and no e thanol activity ascribed to other classes is detectable, supporting the con clusion that evolution of ethanol-active ADH classes by gene duplications o ccurred at early vertebrate radiation after the formation of the amphioxus lineage. Similarly, Southern blot analysis indicated that amphioxus ADH3 is encoded by a single gene present in the methylated fraction of the amphiox us genome and northern blots revealed a single 1.4-kb transcript. In situ e xperiments showed that amphioxus Adh3 expression is restricted to particula r cell types in the embryos. Transcripts were first evident at the neurula stage and then located at the larval ventral region, in the intestinal epit helium. This tissue-specific pattern contrasts with the ubiquitous Adh3 exp ression in mammals.