Glucose-6-phosphate isomerase is not a specific autoantigen in rheumatoid arthritis

Objective. To test the hypothesis that glucose-6-phosphate isomerase (GPI) is a novel autoantigen in RA. Methods. Eighty-eight serum samples from 23 patients with rheumatoid arthritis (RA), 25 with Sjogren's syndrome, 20 with systemic lupus erythematosus and 20 healthy controls were tested by enzyme-linked immunosorbent assay (ELISA) using a commercially available, partially purified rabbit GPI as antigen. Beside each duplicate well containing antigen (10 mug/ml), uncoated blocked duplicate wells (phosphate-buffered saline only) were included as controls for non-specific binding for every serum tested. We also examined antibodies binding to various polypeptides in the GPI preparation by immunoblotting in 73 of the sera. Results. By ELISA, binding levels were low and there was no difference between serum from patients with RA, other rheumatic diseases and normal controls. By immunoblotting, antibodies binding to the GPI polypeptide were present in 70-80% of all groups tested. In addition, we showed that another polypeptide identified as phosphoglucomutase was also present in the preparation and reacted with human immunoglobulins. Conclusion. Our findings suggest that GPI is not a specific autoantigen in RA.