Microheterogeneity of rat alpha-fetoprotein.

A purified and homogeneous preparation of rat AFP, as judged by both electrophoresis on Cellogel and immunoelectrophoresis, was separated into two components, AFPa and AFPb, by disc electrophoresis on 7% polyacrylamide gel. These two components had a definite difference in electrostatic net charge and gave only a single band on SDS-electrophoresis. Immunological reactivity or electrophoretic separation or mobility of the two components could be altered by treatment with either sulfhydryl inhibitors or reducing agents but not by treatment with protein denaturants. Electrophoresis of neuraminidase-treated AFP on 5% polyacrylamide gel yielded clearly separable, slower moving four to six and finally two components depending on the time of incubation with neuraminidase. The time-dependent conversion of faster into slower migrating components of both AFPa and AFPb upon neuraminidase treatment was confirmed by reelectrophoresis of separated and similarly treated AFPa and AFPb. Two bands of sialized or desialized AFP were also observed on isoelectric focusing. Both AFPa and AFPb treated with and without neuraminidase gave single fused precipitin lines against the antiserum in Ouchterlony double-diffusion analysis. On the basis of the changes in electrophoretic mobilities of the intermediates following neuraminidase treatment, AFPa and AFPb were estimated to have at least 2.5 and 4.5 molecules of sialic acid per molecule, respectively.