PURIFICATION AND MOLECULAR CHARACTERIZATION OF THE NAD(+)-DEPENDENT ACETALDEHYDE ALCOHOL-DEHYDROGENASE FROM ENTAMOEBA-HISTOLYTICA

A bifunctional 95 kDa polypeptide (EhADH2) harbouring acetaldehyde dehydrogenase and alcohol dehydrogenase activities was purified to homogeneity from trophozoite extracts of the protozoan parasite Entamoeba histolytica. Kinetic studies revealed that the enzyme utilizes NAD(+) rather than NADP(+) as cofactor. K-m values for acetyl-CoA, acetaldehyde and ethanol were found to be 0.015, 0.15 and 80 mM respectively in the presence of 0.2 mM NAD(+). The primary structure of EhADH2 as deduced from respective amoebic DNA sequences showed striking similarity to the trifunctional AdhE protein of Escherichia coli and the bifunctional AAD protein of Clostridium acetobutylicum. Alignment with a number of aldehyde dehydrogenases and alcohol dehydrogenases from various species suggested that the two catalytic functions of EhADH2 are located on separate parts of the molecule. By cross-linking experiments and electron-microscopic analysis, native EhADH2 was found to be organized in a homopolymeric fashion consisting of more than 20 associated promoters which form rods about 50-120 nm in length.