Comparative study of native proteins in aqueous humor and serum--detection of characteristic aqueous humor proteins.

Native proteins in the aqueous humor and serum from 4 patients with cataract and 2 patients with central artery occlusion were studied using micro two-dimensional isoelectric focusing-gradient gel electrophoresis combined with silver staining. A total of 51 protein spots were detected in the aqueous humor and the standard map of distribution pattern of the native proteins was established. Transferrin, albumin, alpha 2-macroglobulin, ceruloplasmin, haptoglobin, IgA and IgG were identified by an enzyme immunoassay. As a whole, the protein pattern of the aqueous humor is comparable with the pattern of the serum, except for marked quantitative differences. Most aqueous humor components have their corresponding spots found in the serum. However, there are some spots detected only in the aqueous humor but not in the serum. These spots were identified as transferrin. It is already known that the tau fraction (a desialized form of transferrin, absent in serum) is found in an extra band in the electrophoretic patterns of cerebrospinal fluid and vitreous humor. Therefore, the samples were treated with neuraminidase to determine whether or not the different aqueous transferrin spots were the tau fraction. Serum transferrin and certain aqueous transferrin spots (corresponding to serum transferrin in electrophoretic position) were transformed to tau fraction after treatment. However, some other aqueous transferrin spots (tau fraction and unreported transferrin) remained unchanged. This indicated that, in the aqueous humor, there are three kinds of transferrin molecules: ordinary serum transferrin, tau fraction and characteristic aqueous humor transferrin. Soluble proteins in the extracts of ciliary processes, iris, vitreous humor, sensory retina and lens were also studied. The characteristic transferrin pattern seen in the aqueous humor was observed only in the vitreous humor. The ciliary processes and iris revealed an identical transferrin pattern as in the serum. alpha 2-Macroglobulin, that has been thought to be too large to pass the blood-aqueous barrier, was detected in all of the aqueous humor samples. Certain larger serum proteins were observed to have their corresponding spots in the aqueous, whereas certain smaller ones were not found. These findings strongly suggested that the aqueous humor proteins are not a simple ultrafiltrate of the serum. Active transport and/or local synthesis of proteins may play important roles in determining the constitution of the proteins in the aqueous humor.