An absorption band at 340 nm is shown to be formed concomitantly with the covalent bond between the affinity label 3-chloroacetylpyridine -adenine dinucleotide (clac3PdAD ') and glyceraldehyde-3-phosphate from sturgeon. This band corresponds to a charge-transfer transition. Its intensity depends upon the pH and the ionic strength but is almost independent of the nature of the anions present in the medium. The pH dependence shows an inflexion point at pH 7.1. This result suggests the participation of a residue with a pK, of 7.1 within the active site of the enzyme in the formation of this transition. Using various techniques, the amino acid alkylated by clac3PdAD+ is shown to be the essential Cys-149, thus excluding the participation of this residue in the formation of the charge-transfer transition. On the other hand, the modification of Cys-153 seems not to affect this charge-transfer band. Other possible donors are proposed, such as the invariant His-176 or Tyr-317 residues. These amino acids might be implicated in the formation of the Racker band.In a previous paper, 3-chloroacetylpyridine -adenine dinucleotide was shown to inactivate glyceraldehyde-3-phosphate dehydrogenase from Bacillus stearothermophilus and sturgeon muscle via an affinity label mechanism [I]. The labeling of both glyceraldehyde-3-phosphate dehydrogenases was demonstrated to be irreversible, with a covalent incorporation of one mole of inactivator/mole enzyme subunit [I].The pH dependence of the inactivation rate and the results on thiol titration under denaturating conditions were rather indicative of a modification of a cysteine residue [l]. More surprising were the results on thiol titration under native conditions which showed the accessibility of one Cys residue per subunit toward 5,5'-dithiobis(2-nitrobenzoate).This result suggested that the modification of a Cys, presumably Cys-349, could lead to a partial unfolding of the structure of the protein. As a consequence, other thiol residues would then become accessible to thiol reagents.Results presented in this paper show that an absorption band at 340 nm is formed concomitantly with the covalent bond between 3-chloroacetylpyridine -adenine dinucleotide and glyceraldehyde-3-phosphate dehydrogenase from sturgeon. The properties of this new absorption band raised the question of the true nature of the amino acid alkylated by clac3PdADC. For this reason using various techniques but in particular high-performance liquid chromatography (HPLC), we have determined the nature of the modified amino acid.Abbreviations. ac'PdAD ', ac3PdADH, 3-acetylpyridine -adenine dinucleotide, oxidised and reduced forms respectively; clac3PdAD+, clac3PdADH, 3-chloroacetylpyridine -adenine dinucleotide, oxidised and reduced forms respectively; alkylated enzyme, ~-glycerdldehyde-3-phosphate dehydrogenase fully alkylated by clac3PdAD+ ; Tes, 2-{ [2-hydroxy-I ,I -bis(hydroxymeth yl)ethyl]amino}ethanesulfonic acid ; HPLC, high-performance liquid chromatography.Enzymes. D-Glyceraldehyde-3-phosphate dehydrogenase or ...