THE 64-KILODALTON EYE MUSCLE PROTEIN IS THE FLAVOPROTEIN SUBUNIT OF MITOCHONDRIAL SUCCINATE-DEHYDROGENASE - THE CORRESPONDING SERUM ANTIBODIES ARE GOOD MARKERS OF AN IMMUNE-MEDIATED DAMAGE TO THE EYE MUSCLE INPATIENTS WITH GRAVES HYPERTHYROIDISM

Thyroid-associated ophthalmopathy (TAO) is a progressive eye disorder associated with thyroid autoimmunity, particularly Graves' hyperthyroi dism, which is generally considered to have an autoimmune etiology. Ey e muscle membrane proteins reportedly of 55 and 64 kDa are the best ma rkers of the ophthalmopathy. The main focus of our recent studies has been to purify the pertinent proteins from porcine eye muscle membrane s and characterize them. The 64-kDa protein is now shown from a partia l sequence and by Western blotting using specific antibody probes to b e the flavoprotein (Fp) subunit of succinate dehydrogenase and to have a correct molecular mass of 67 kDa. The protein was purified and clea ved with cyanogen bromide, and the N-terminal region of an immunoreact ive partial peptide was determined. The 20-amino acid porcine sequence so obtained matched one within the Fp subunits of human and bovine su ccinate dehydrogenases in 20 and 18 of these positions, respectively. Succinate dehydrogenase:is both a citric acid cycle enzyme and a compo nent (complex II) of the mitochondrial respiratory chain. It is thus e ssential for aerobic energy production and is highly conserved. The ma ture human and bovine Fp subunits are 92% homologous and have a molecu lar mass of similar to 67 kDa, the same as our redetermined value for the 64-kDa marker protein. Sera from patients with TAO and from those with Graves' hyperthyroidism without evident ophthalmopathy highlighte d the 64-kDa marker protein in crude porcine eye muscle membranes and the Fp subunit of highly purified bovine succinate dehydrogenase at th e identical position on Western blots. Anti-beef Fp antibodies were de tected in sera from 67% of patients with active TAO of more than l-yr duration, in 30% with stable TAO of more than 3-yr duration, and in 30 % of patients with Graves' hyperthyroidism without ophthalmopathy, but in only 7% of age-and sex-matched normal subjects. As succinate dehyd rogenase is bound to the matrix (inside) surface of the mitochondrial inner membrane, it is unlikely to be accessible to circulating autoant ibodies. We would postulate that eye muscle damage in ophthalmopathy i s probably caused by cytotoxic antibodies or CD+ T lymphocytes targeti ng a cell membrane antigen, such as the thyroid and eye muscle shared protein G2s, and that presentation of succinate dehydrogenase is secon dary. On the other hand, an autoantibody response to succinate dehydro genase may be a good marker of immune-mediated damage to the eye muscl e fiber and may support the idea that the extraocular muscles are targ ets of the autoimmune reactions of TAO.