Two types (isoenzymes) of octopine dehydrogenase (A and B) from Pecten jacobaeus adductor muscle were purified to homogeneity, applying affinity chromatography as an efficient final step of purification. Both forms of the enzyme differ in their electrophoretic mobility. All other physico-chemical and enzymatic properties, as well as the folding behaviour were found to be identical. Interconversion of one form into the other was not detectable. Sedimentation equilibrium, gel permeation chromatography, and NaDodSO4/polyacrylamide gel electrophoresis yield a relative molecular mass of 45000 +/- 1500 for both native and denatured enzyme. The unfolding transition at varying guanidine X HCl concentrations is characterized by a two-step profile: at 0.4-0.8 M, partial unfolding is parallelled by inactivation; at 2.0-2.4 M the residual structure is destroyed in a second unfolding step. Beyond 2.8 M no further changes in fluorescence emission and dichroic absorption are observed. At 0.4-1.8 M guanidine X HCl, partial unfolding is superimposed by aggregation. The emission maximum of the intrinsic protein fluorescence at 327 nm is shifted to 352 nm upon denaturation in 6 M guanidine X HCl. Changes in the far-ultraviolet circular dichroism indicate complete loss of the overall backbone structure in this denaturant, including the native helix content of about 33%. Denaturation in 6 M guanidine X HCl, as monitored by the decrease of protein fluorescence, is fast (less than 8s). Upon reactivation after short denaturation, about 25% of the activity is recovered in a fast initial phase (less than 20s). The product of this phase has a similar stability towards destabilizing additives or proteases as the native enzyme. The slow phase of reactivation, which predominates after long-term denaturation, is determined by a single first-order reaction characterized by tau = 29 +/- 3 min (20 degrees C). This reaction must be a relatively late event on the folding pathway, preceded by the fast formation of a structured intermediate, as indicated by the immediate recovery of the native fluorescence. The structural rearrangements, which are rate-limiting for reactivation after long-term denaturation, are characterized by a high energy of activation (112 +/- 8 kJ/mol). The slow reactivation step is compatible in rate with the first-order folding reactions involved in the reconstitution of several oligomeric dehydrogenases [c.f. R. Jaenicke and R. Rudolph (1983) Colloq. Ges. Biol. Chem. Mosbach 34, 62-90].