Spectroscopic characterization of the number and type of iron-sulfur clusters in NADH:ubiquinone oxidoreductase.

The number and type of iron-sulfur clusters present in the NADH dehydrogenase of the mammalian respiratory chain were studied by a combination of low temperature magnetic circular dichroism (MCD) and quantitative electron paramagnetic resonance spectroscopies. MCD was used with the high molecular weight, soluble enzyme, and EPR was used with both the purified enzyme and Complex I (NADH:ubiquinone oxidoreductase). The results of the EPR experiments of the two types of preparations agreed with each other, as well as with the data in the literature for various types of membrane-bound preparations. The two methods gave concordant results showing the presence of one binuclear and of three tetranuclear NADH-reducible iron-sulfur clusters. Earlier studies using the cluster extrusion technique indicated a higher ratio of binuclear to tetranuclear clusters which may be explained by cluster interconversion during the extrusion process.