Identification of the high‐molecular‐mass mitochondrial oxaloacetate keto‐enol tautomerase as inactive aconitase

The homogeneous bovine heart mitochondrial high‐molecular‐mass oxaloacetate keto‐enol tautomerase [(1988) Biochim. Biophys. Acta 936, 10–19] is shown to be an iron‐sulfur protein as revealed by the enzyme spectral properties and direct chemical determination of non‐heme iron and acid‐labile sulfur. The protein is capable of catalysing the aconitase reaction after treatment with ferrous ions under anaerobic conditions. Treatment of the ‘activated’ protein with N ‐ethylmaleimide results in the simultaneous irreversible loss of the oxaloacetate keto‐enol tautomerase and aconitase activities. The effects of some substrates and inhibitors on both activities show that the same catalytic site is involved in the oxaloacetate tautomerase and aconitase reactions. It is concluded that the protein previously described as a 80 kDa oxaloacetate keto‐enol tautomerase is inactive aconitase.