The rapid inactivation of 3-HBA-6-hydroxylase by 100 mu M diethylpyrocarbonate or 40 mu M N-bromosuccinimide and protection offered by the substrate, 3-hydroxybenzoate, against these chemical modifications implicate the involvement of histidine and tryptophan in the catalytic activity of the enzyme. Inactivation of the enzyme by diethylpyrocarbonate followed pseudo-first-order kinetics, and an "n" value of 1.3 was obtained. Inactivation of the enzyme by N-bromosuccinimide was instantaneous and failed to follow pseudo-first-order kinetics. Distinct and incremental changes in the UV absorption, emission fluorescence, and near UV-CD spectra of the enzyme upon its titration with increasing concentrations of diethylpyrocarbonate or N-bromosuccinimide may be ascribed to modification and/or changes in the microenvironment of aromatic amino acid residue(s) such as tryptophan in the enzyme.