Isolation and characterization of modified forms of soybean leghemoglobin a

Three modified forms of leghemoglobin a (Lbams) were resolved and purified by preparative isoelectric focusing. In the presence of nicotinate, their pI values were different from those of the nicotinate complexes of ferric Lba and Lbb. The visible spectra of ferric Lb and ferric Lb nicotinate complex were completely different from those of Lba. Lbams could be reduced to their ferrous forms and the resulting ferrous nicotinate complexes exhibited spectra identical to that of the analogous nicotinate complex of Lba. Lbams did not appear to form a stable complexes with oxygen or carbon monoxide in the presence of dithionite or a reducing system containing NADH and riboflavin in oxygen or carbon monoxide saturated buffer, indicating that Lbams are nonfunctional Lbs. Total amino acid compositions, N-terminal sequences and C-terminal sequences of the three Lbams were identical to those of Lba, indicating that the apoproteins of Lbams were not altered by proteolysis. The pyridine hemochromogen absorption spectra revealed that the hemes of Lbams were perturbed. However, $\alpha$- and $\beta$-bands of the absorption spectra were apparently observed in the Lbams. These results suggested that their hemes were modified without cleavage of the porphyrin ring or loss of iron. EPR spectra of the ferric Lbams and the ferric Lbam nicotinate complexes indicated that certain low spin signals were changed, compared to those of Lba. FTIR spectra of ferric Lbams, also, were different from those of ferric Lba. These results indicate that there are perturbations in heme environment due to the modification of heme. Since the porphyrin ring appeared intact and contained iron, hemes in the Lbams might be modified by enzymatic or nonenzymatic inactivation of Lba. The minor structural changes that give rise to the modified forms of the Lbs may represent very early steps in the turnover pathway of Lb