DTNB modification of SH groups of isocitrate dehydrogenase from Bacillus stearothermophilus purified by affinity chromatography

A new method of affinity chromatography using blue dextran-Sepharose 4B resin was established to purify NADP+-dependent isocitrate dehydrogenase [EC 1.1.1.42] from Bacillus stearothermophilus in high yield. The purified preparation was found to be homogeneous on disc gel electrophoresis. The SH groups of the Enzyme were modified with 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) to determine the number of SH groups per molecule and their contribution to the Enzyme activity. One SH group was titrated with DTNB per subunit (the native Enzyme consisted of two subunits) and after complete denaturation with 4 M guanidine-HCl the number of titratable SH groups remained unchanged. ORD and CD measurements showed that the alpha-helical conformation of the polypeptide backbone was unaffected by DTNB modification, though the near ultraviolet CD spectrum was evidently altered. The fluorescence derived from tryptophanyl residue(s) was quenched by the modification to 30% of the native level, which may indicate the presence of SH in the vicinity of tryptophanyl residue(s). A remarkable decrease of the Enzyme activity was detected upon modification with DTNB, but there was some discrepancy between the rate of inactivation and that of modification of SH groups. The presence of substrate and Mg2+ gave partial protection against modification of the SH groups by DTNB. Complete protection of the native Enzyme activity against heating at 65 degrees was observed in the presence of substrate and Mg2+, but the thermostability of the Enzyme was markedly reduced by modification of the SH groups.