6-Phosphogluconate dehydrogenase (6-phospho-D-gluconate:NADP oxidoreductase (decarboxylating), EC 1.1.1.44) of Escherichia coli MREp 600 has been isolated with the purpose of carrying out comparative studies with the thermostable enzyme previously isolated from Bacillus stearothermophilus (Veronese, F.M., Boccù, E.,Fontana, A., Benassi,C.A., and Scoffone, E. (1974), Biochim, Biophys. Acta 334, 31). The purified enzyme appeared homogeneous by the criteria of disc gel electrophoresis with and without sodium dodecyl sulfate, ultracentrifugation, and gel filtration. The enzyme has enzymological and physiochemical properties similar to the enzyme isolated from other sources, including B. stearothermophilus. The E. coli enzyme has a mol wt of 100,000 +/- 3000 and is composed of two apparently identical subunits. The amino acid composition of both the mesophilic and thermophilic enzyme has been determined and found to present large similarities. The E. coli enzyme shows a high degree of specificity for nicotinamide adenine dinucleotide (NADP) and it is inhibited by reduced NADP (NADPH). Cysteine residues are involved in the catalytic activity, since on incubation of the enzyme with p-chloromercuribenzoate or 5.5'-dithiobis(2-nitrobenzoic acid) strong inhibition occurs, activity being restored by treatment with excess of beta-mercaptoethanol. The substrate 6-phosphogluconate protects partially the enzyme from inactivation. Both the mesophilic and thermophilic 6-phosphogluconate dehydrogenases are inactivated by Rose Bengal in the presence of light by similar kinetics and protected against photoinactivation by the enzyme substrate. The E. coli enzyme, on the other hand, showed distinct differences in stability against heat and unfolding agents in respect to the B. stearothermophilus enzyme. Heating at 50 degrees C or incubation in 8 M urea results in rapid inactivation. The gross structure of the mesophilic and thermophilic enzyme was very similar as judged by circular dichroic measurements. The far-ultraviolet circular dichroic spectrum had a negative band centered at about 220 nm. In both cases, the fluorescence emission spectrum indicates that the environment of the tryptophan residues is similar, since both enzymes show an emission maximum at 334 nm upon excitation at 295 nm. Circular dichroism measured at various temperatures between 25 and 80 degrees C showed the mesophilic enzyme to be conformationally stable below about 45 degrees C and the thermophilic enzyme below 60 degrees C. The secondary structure of the E. coli enzyme was very sensitive to the denaturing action of urea, since in 8 M urea it rapidly unfolded. Partial renaturation after urea treatment occurred on dilution with buffer or dialysis, as evidenced by spectral properties of the renatured enzyme. The results show that the mesophilic and thermophilic enzymes are very similar and that differences in thermal stability depend on subtle differences in the architectures of the proteins.