Sorbitol dehydrogenase of Drosophila - Gene, protein, and expression data show a two-gene system

The Drosophila melanogaster sorbitol dehydrogenase (SDH) is characterized a s a two-enzyme system of the medium chain dehydrogenase/reductase family (M DR). The SDH-1 enzyme has an enzymology with K-m and k(cat) values an order of magnitude higher than those for the human enzyme but with a similar K-c at/K-m ratio. It is a tetramer with identical subunits of similar to 38 kDa . At the genomic level, two genes, Sdh-1 and Sdh-2, have a single transcrip tional start site and no functional TATA box. Expression is greater in larv ae and adults than in pupae, where it is very low. At all three stages, Sdh -1. constitutes the major transcript. Sdh-1 and Sdh-2 genes were located at positions 84E-F and 86D in polytene chromosomes. The deduced amino acid se quences of the two genes show 90% residue identity. Evaluation of the seque nce and modeling of the structure toward that of class I alcohol dehydrogen ase (ADH) show altered loop and gap arrangements as in mammalian SDH and es tablishes that SDH, despite gene multiplicity and larger variability than t he "constant" ADH of class III, is an enzyme conserved over wide ranges.