Sera from 29 of 48 patients with idiopathic dilated cardiomyopathy (IDCM) and six of six patients with dilated cardiomyopathy (DCM) secondary to suspected viral myocarditis were shown to react with the branched chain alpha-ketoacid dehydrogenase (BCKD) complex mitochondrial proteins. Whereas sera from only 1 of 26 patients with ischemic heart disease showed reactivity against the BCKD complex protein, 0 of 30 sera from normal human volunteers, 0 of 64 sera from patients with lupus, and 0 of 34 sera from patients with rheumatoid arthritis showed detectable reactivity, denoting an element of specificity for the reactivity of sera from IDCM patients. The major reactivity was localized to the dihydrolipoyl transacylase (E2) component of BCKD complex. By using recombinant techniques, the immunodominant BCKD-E2 epitope recognized by sera from IDCM patients was localized to amino acid (aa) sequences 116 to 134. Each of the IDCM sera that reacted with the native BCKD complex was shown to react with the immunodominant peptide, as defined by a peptide inhibition ELISA and by an ELISA using the reactive peptide conjugated to BSA. Sera from IDCM patients that reacted with the native BCKD complex and the reactive peptide also showed inhibition of BCKD enzyme activity. The possible mechanisms for the induction of the Abs and the implications of these findings for the pathogenesis of IDCM are discussed.

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