Different forms of molybdenum cofactor in Vicia faba seeds: The presence of molybdenum cofactor carrier protein and its purification

There were significant differences in the contents of molybdenum cofactor (Mo-co), both in a low-molecular-mass form (free Mo-co) and in a protein-bound form, in seeds of seven Vicia faba genotypes. Low-molecular-mass Mo-co species present in the extracts were detected by their ability to reactivate, through a dialysis membrane, aponitrate reductase from the Neurospora crassa nit-1 mutant. In extracts of all genotypes tested, the amount of Mo-co capable of directly reactivating nitrate reductase of the N. crassa nit-1 mutant was always much higher than that of low-molecular-mass Mo-co. These data cannot be explained by considering, as traditionally, that Mo-co detected directly, i.e. without any previous treatment for its release from Mo-co-proteins, corresponds to free low-molecular mass Mo-co. A protein which bound Mo-co was purified to electrophoretic homogeneity. This protein consisted of a single 70-kDa polypeptide chain and carried a Mo-co that could be efficiently released when in contact with aponitrate reductase.