Glucose-6-phosphate dehydrogenase from Saccharomyces cerevisiae (bakers' yeast) reacts with acetylsalicylic acid, and this is accompanied by inactivation and modification of essentially one lysine residue per subunit. The amino acid sequence of an 11-residue tryptic peptide containing the reactive lysine residue of the yeast enzyme is given and establishes the existence of different subgroups of glucose-6-phosphate dehydrogenases. Thus, the labeled yeast structure has few similarities to the known structure around the reactive lysine residue of the enzyme from Leuconostoc mesenteroides, although it has extensive similarities with a structure in the human enzyme. It is further shown that amino acid sequences around reactive lysine residues of dehydrogenases in general vary, even though similarities occur around reactive lysine residues in 6-phosphogluconate, glutamate, and glyceraldehyde-3-phosphate dehydrogenases.