Salivary proteins in rheumatoid arthritis and Sjögren's syndrome: one-dimensional and two-dimensional electrophoretic studies

Parotid saliva from patients with rheumatoid arthritis and Sjögren's syndrome contains elevated levels of multiple anionic proteins of pI approximately 3.75-4.75 and Mr approximately 32,000. Further studies on these components involving narrow range pH 3.5-5.0 immobilized pH gradients (IPGs) and two-dimensional (2D) electrophoresis with narrow- or broad-range IPGs in the first dimension have confirmed their association with these disorders. Immunoblotting showed that these proteins include multiple forms of tissue kallikrein. Treatment with neuraminidase results in removal of these anionic substances from the pH 3.75-4.75 region of gels, thereby indicating that heterogeneity arises from differences in sialation of the carbohydrate residues. The results of treatment with endo-beta-N-acetylglucosaminidase (Endo F) or peptide N-glycosidase (PNGase F) and comparison of the results with studies on human urinary kallikrein suggest that proteins other than kallikrein may also comigrate in the anionic region of gels and that deglycosylation of kallikrien is incomplete in the experimental conditions used, probably because of inadequate denaturation. The paper also reviews the limitations of current criteria used in the investigation of salivary gland function associated with connective tissue disorders and the diagnosis of these. It assesses the potential of electrophoresis in forming the basis of new diagnostic techniques and furthering the understanding of the nature of these diseases. The findings presented in this paper could make a key contribution to this.