Tau fraction of transferrin is present in human aqueous humor and is not unique to cerebrospinal fluid.

In view of the many similarities between the aqueous humor and the cerebrospinal fluid (CSF), we investigated whether the tau fraction of transferrin, which is believed to be specific for CSF, is also present in aqueous humor. Samples of aqueous humor and CSF from clinically normal individuals were analyzed by SDS-PAGE and isoelectric focusing (IEF), and probed with anti-transferrin antiserum and the lectin Limax flavus agglutinin. Specific bands which corresponded to transferrin and its tau fraction were detected in both fluids. After isolation of these bands by affinity chromatography with anti-transferrin antiserum, we detected a single, diffuse fraction at an apparent molecular weight of 80 kDa in both aqueous humor and CSF. Peptide mapping revealed no detectable difference in this fraction between the two fluids. The detection of the tau fraction in human aqueous humor indicates that this molecule cannot be considered a specific cerebrogenic marker as had been thought previously. In view of the known growth-promoting properties of transferrin, its isolation from both ocular and cranial fluids attains special significance.