A mitochondrial iron protein with properties of a high-potential iron-sulfur protein

Abstract

A brown, soluble protein which, in its oxidized form, has a highly temperature sensitive EPR signal centered at g=2.01 with a width of 25 gauss at 9.2 GHz, was purified from the supernatant of sonicated beef heart mitochondria to apparent homogeneity by criteria of protein chemistry. No other signals were observed in the oxidized or reduced state. Two g atoms each of iron and labile sulfur were found per mole (MW 97,000). Signal intensity and reductive titration account for 3 electrons per molecule. The protein is labile and autooxidizable. In view of the stoichiometric presence of iron and labile sulfur and the magnetic properties, it is proposed that this protein belongs to the class of high-potential iron-sulfur proteins. Heterogeneity in the state and distribution of iron-sulfur groups, but not at the level of gross protein structure, is assumed to explain the observed stoichiometry. On fractionation of mitochondria an EPR signal similar to that of the purified protein is concentrated in succinate-ubiquinone reductase. Aspects of the function of the corresponding substance in electron transfer are dealt with in a companion report (Beinert, Ackrell, Kearney, and Singer, Biochem. Biophys. Res. Commun. this issue).