Comparative properties of bovine heart and liver rhodaneses and the regulatory role of the rhodaneses in energy metabolism

In previous studies on the rhodanese activity of bovine liver mitochondria, we have shown that in addition to activity observed in the soluble protein fraction, there is rhodanese activity that is bound to the mitochondrial membrane. The latter activity accounts for as much as 40% of the total and, in situ, is associated in a multiprotein complex that forms iron-sulfur centers. In the present studies, we have investigated the rhodanese activity of bovine heart muscle. We have found that the major part of this enzyme activity is localized in the mitochondria and, further, that at least 25% of the total rhodanese activity of heart mitochondria is membrane-bound. As in liver tissue, the heart activity at least in part is associated in a multiprotein complex that forms iron-sulfur centers. Upon purification of the heart rhodanese in the soluble protein fraction, there is a 10- to 30-fold decrease inK _m values for the standard assay substrates thiosulfate and cyanide ions. These observations are consistent with the interpretation that there are activated and deactivated (low activity) forms of the heart enzyme in crude extracts, but only the activated form survives purification. The present results, together with our recent finding that liver mitochondrial rhodanese is subject to phosphorylation, lend support to our proposal that the rhodaneses serve as converter enzymes which regulate the rate of electron transport through sulfuration of respiratory chain components. The rhodaneses, in turn, are controlled by protein kinases and the local ATP concentration.