Transferrin isoelectric point

The transferrins contain varying amounts of carbohydrates (Table 7) and can be classified as glycoproteins. Williams (136) stated that ovotrans-ferrin and chicken serum transferrin differed only in carbohydrate contents. The ovotransferrin contains no sialic acid (44, 136). Serum transferrin, on the other hand, contains sialic acid. Williams (136), from the results of treating the serum transferrin with the enzyme neuraminidase, suggested that differences in the electrophoretic mobilities and isoelectric points of the multimolecular forms of serum transferrins may be due to differences in their contents of sialic acid. 

Extensive studies have been made of the charge relationships and electrostatic properties of ovotransferrin, and a few studies have been made on other transferrins. Several of these primarily concern the changes seen on chelation of metal ions, which will be discussed below. Most of the transferrins have isoelectric points around pH 6 (Fig. 2), but the isoelectric point may vary considerably with species and may show taxonomic relationships (28). These will be discussed below under genetic relationships. 

The isoelectric point of transferrin depends on its content of metal, since the binding of each metal ion increases the net negative charge of the protein according to the equation (23, 24) ... 

Human transferrin displays genetic polymorphism with transferrin C being the most common phenotype in all populations, whereas allelic B (lower isoelectric point, p7) and D (higher p7) variants, with a different primary structure but a normal set of glycans, occur at lower frequencies. ... 

Near its isoelectric point, binding of iron to transferrin results in the introduction of a negative charge due to uptake of one bicarbonate ion by the molecule. Aisen et al. (86) first showed this effect when an increase in the anodic electrophoretic mobility of the protein was correlated with its extent of iron saturation. The observation has been corroborated by isoelectric fractionation experiments (87). It is curious that although DEAE-cellulose chromatography of partially iron-saturated transferrin yields three peaks, as would be expected from the three species present in the mixture, the 1 Fe-transferrin eluted first, followed successively by the iron-saturated molecule and the apoprotein (88). Thus the order of elution does not agree with the expectations based on the net electric... 

Unlike Tf derived from serum, Tf from bloodstains present a secondary band for each allelic product. Transferrin has two iron binding sites and the monoferric form of Tf is the most stable. It appears that in bloodstains either binding site is available for iron. Depending upon the site occupied by iron the monoferric forms will have slightly different isoelectric points, and both monoferric forms will exist within a bloodstain. Thus, the monoferric form of Tf will produce two protein bands for each allelic product observed after ULPAGIF. This hypothesis is supported by the fact that these secondary bands do... 

---