Anti-mitochondrial flavoprotein autoantibodies of patients with myocarditis and dilated cardiomyopathy (anti-M7): interaction with flavin-carrying proteins, effect of vitamin B2 and epitope mapping

Vitamin B2 and flavin cofactors are transported tightly bound to immunoglob ulin in human serum. We reasoned that anti-mitochondrial flavoprotein autoa ntibodies (alpha Fp-AB) present in the serum of patients with myocarditis a nd cardiomyopathy of unknown aetiology may form immunoglobulin aggregates w ith these serum proteins. However, immunodiffusion and Western blot assays demonstrated that the flavin-carrying proteins were not recognized by alpha Fp-AB. Apparently the flavin moiety in the native protein conformation was inaccessible to alpha Fp-AB. This conclusion was supported by the absence of an immunoreaction between the riboflavin-binding protein from egg white and alpha FP-AB. Intravenous application of vitamin B2 to rabbits immunized with 6-hydroxy-D-nicotine oxidase, a bacterial protein carrying covalently attached FAD, did not neutralize alpha Fp-AB which had been raised in the serum of the animals. FAD-carrying peptides generated from 6-hydroxy-D-nico tine oxidase by trypsin and chymotrypsin treatment were not recognized by t he alpha Fp-AB, but those generated by endopeptidase Lys were. This demonst rates that the epitope recognized by alpha Fp-AB comprises, besides the fla vin moiety, protein secondary structure elements.