Conformational Variants of Human Alpha-Fetoprotein

Abstract

The immunological heterogeneity of human α-fetoprotein (AFP) was demonstrated using immunoaffinity electrochromatography on monoclonal antibodies (MoAbs) to 3 non-cross-reacting epitopes of this protein. At least 4 subfractions expressing different epitopes were found in the native AFP. These subfractions demonstrated molecular weights similar to the major component of the original AFP. The difference between epitope F₅-positive and F₅-negative subfractions disappeared when epitope-negative subfraction was conformationally changed after fixation onto nitrocellulose membrane (NCM). Thus, the epitope under study exists in two forms on the native human AFP molecule: an open and a cryptic form. The cryptic form could be revealed after partial denaturation by fixation on NCM. The epitope variants of AFP could possess different functions in multifunctional AFP. The AFP epitope heterogeneity found in this work should be taken into account when constructing diagnostic AFP kits and when isolating purified AFP using anti-AFP MoAbs.

Epitope Mapping of Human Alpha-Fetoprotein

“…All these findings strongly suggest the cryptic nature of cluster D epitope. The full proof of this suggestion is given in the accompanying paper [12].…”

“…The former type of competition was very clearly expressed between clusters A and D. One possible explanation of unidirectional cross-reactivity could be MoAbs' identical specificity but significant difference in the affintiy to the same epitope. In this case it might be expected that the high-affinity The partial competition phenomenon which was observed with certain MoAb pairs could be explained by the existence of the epitope variants in AFP [12]. In this case, the MoAb would inhibit only a fraction of AFP possessing the corresponding epitope.…”

New Approaches for the Detection and Characterization of Alpha-Fetoprotein Epitope Variants

“…Earlier we proposed an approach for the separation of proteins into fractions based on the expression of individ- ual epitopes in the native molecule, which also demonstrated that AFP preparations contained several epitope variants [10,11]. Since the AFP structure is determined by a single gene representing a single polypeptide chain with varying oligosaccharide moieties [3], these different epitope variants would appear to be conformational ones of the same polypeptide chain.…”

“…Since the AFP structure is determined by a single gene representing a single polypeptide chain with varying oligosaccharide moieties [3], these different epitope variants would appear to be conformational ones of the same polypeptide chain. Indeed, one of the AFP epitopes was shown to exist in both an antibody accessible form (defined as the epitope + or ep + variant), and in a cryptic form (defined as the epitope -or ep -variant) [11].…”

Summary Report: Epitope Analysis of Human Alpha-Fetoprotein

“…These results were almost identical to that found by Dr. Nustad et al [17] and Kamakura [18] found that one of the epitopes did apparently contain carbohydrate in their studies. Dr. Karamova et al [19] …”