Drosophila melanogaster aldolase: Characterization of the isozymes α, β, and γ generated from a single gene

Three isozymic forms, α, β, and γ, of Drosophila melanogaster aldolase are produced from a single gene by alternative usage of the triple exons 4 (4^α, 4^β, and 4^γ) (Shaw-Lee et al. (1992) J.Biol. Chem. 267, 3959-3967; Kim et al. (1992) Mol. Cell. Biol. 12, 773-783; Kai et al. (1992) J. Biochem.112, 677-688). The expression plasmids for the respective isozymes were transfected into Escherichia coli cells, and the isozymes α and β were purified to homogeneity by a simple procedure, though isozyme γ was only partially purified. These isozymes are active towards two substrates, fructose-1, 6-bisphosphate (Fru-1, 6-P₂) and fructose-1-phosphate (Fru-1-P), with a preference for Fru-1,6-P₂ over Fru-1-P, but they have different k_cat/K_m values towards these two substrates; isozyme α shows the highest k_cat/K_m value for Fru-1-P among the three isozymes, whereas isozyme β has the highest value for Fru-1,6-P₂. These isozymes show similarity in optimal pHs, thermalstability, and K_m values for both Fru-1,6-P₂ and Fru-1-P. They are composed of four identical subunits of 40 kDa, forming a tetramer with a molecular weight of approximately 160 kDa. The three isozymes are different in primary structure only at the carboxyl-terminal region encoded by the respective exon 4. Therefore, this region should be primarily responsible for the distinct characteristics of these isozymes.