New and previously published data on a variety of ThDP-dependent enzymes such as baker's yeast transketolase, yeast pyruvate decarboxylase and pyruvate dehydrogenase from pigeon breast muscle, bovine heart, bovine kidney, Neisseria meningitidis and E. coli show their spectral sensitivity to ThDP binding. Although ThDP-induced spectral changes are different for different enzymes, their universal origin is suggested as being caused by the intrinsic absorption of the pyrimidine ring of ThDP, bound in different tautomeric forms with different enzymes. Non-enzymatic models with pyrimidine-like compounds indicate that the specific protein environment of the aminopyrimidine ring of ThDP determines its tautomeric form and therefore the changeable features of the inducible effect. A polar environment causes the prevalence of the aminopyrimidine tautomeric form (short wavelength region is affected). For stabilization of the iminopyrimidine tautomeric form (both short- and long-wavelength regions are affected) two factors appear essential: (i) a nonpolar environment and (ii) a conservative carboxyl group of a specific glutamate residue interacting with the N1' atom of the aminopyrimidine ring. The two types of optical effect depend in a different way upon the pH, in full accordance with the hypothesis tested. From these studies it is concluded that the inducible optical rotation results from interaction of the aminopyrimidine ring with its asymmetric environment and is defined by the protonation state of N1' and the 4'-nitrogen.