The Effect of Urea on the Activity and Stability of Metallocluster of the MoFe Protein of Nitrogenase from Azotobacter vinelandii

When the MoFe protein from Azotobacter vinelandii was treated with more than 0.5 mol/L of urea anaerobically, the C2H2-reduction activity of the treated protein was exponentially decreased. However, the activity could be significantly restored after the treated protein was diluted with the buffer system and followed by incubation at 15 ℃. The urea had remarkable enhancement on chelation of Fe atoms from the reducted MoFe protein and the P-cluster-deficient MoFe protein by O-phenanthroline (O-phen), respectively. The migration of the reducted MoFe protein on the urea-gradient gel electrophoresis did not significantly change from 0 to 1.5 mol/L urea , linearly became smaller from 1.5 to 5.0 mol/L urea, and reached a stable state from 5.0 to 8.0 mol/L urea. The results indicated that: (1) The effect of urea on the activity and the stability of metallocluster of the MoFe protoin was mainly attributed to the conformational change of the protein in urea, moveover, the effect of urea on the metallocluster might not be all the same if the states of MoFe protein were dif ferent; (2) There was a close relationship between the conformation and the metalloclusters of MoFe protein; (3) In the course of the denaturation, the decrease in the activity of MoFe protein probably happened prior to the conformational change of the whole molecule.     

Some modification in the assay of Fe2+ in 1–10, o-phenanthroline extracts of fresh plant tissues

Summary The over-estimation of Fe²⁺ by a spectrophotometer due to simultaneous extraction of certain colouring matter in o-phenanthroline extracts can be avoided by making determinations on an atomic absorption spectrophotometer. It is shown that estimates of iron by atomic absorption spectrophotometer did not include measurable amount of Fe³⁺ due to a high specificity of o-phenanthroline for plant Fe²⁺.