Rat alpha‐fetoprotein heterogeneity

The interaction between Ricinus communis agglutinin (RCA) with whole alpha-fetoprotein (AFP) and with its electrophoretic variants by affinity chromatography using Sepharose coupled lectin has been examined. AFP was isolated from amniotic fluid fetal newborn sera and hepatoma bearing rat serum and was purified by affinity chromatography on anti AFP-Sepharose column. The data demonstrate that RCA1-reactive AFPA2 had a lower content of sialic acid (4 residues/mole) as compared to the RCA1 nonreactive AFPA1 (6 residues/mole). It is postulated that the lack of 2 N-acetylneuraminic acid residues exposes 2 D-galactose terminal sites in the reactive form allowing it to react with the Ricinus lectin. This observation was confirmed by enzymic desialation of AFPA which unmasks all D-galactopyranosyl sites and allows the quantitative binding of the desialated variant. AFPB glycans appeared to be less accessible as shown by their incomplete in vitro enzymic desialation and their nonreactivity with the Ricinus lectin. AFP1 is invariably found in fresh sera or adult hepatoma sera and is not an artefact due to the method of isolation. Dosage of these proteins in the serum of rats from birth until Day 24 revealed that AFPB and AFPA2 disappear faster than the AFPA1 which may signify either that they become preferentially localized in certain organs or that they are more rapidly catabolized in the sera.