Behavior of Human Serum R-Type Vitamin B 12 -Binding Proteins (Cobalophilins) on Lectin-Sepharose Affinity Resins

Abstract

Abstract

R-Type serum vitamin B₁₂-binding proteins (cobalophilins) are a heterogenous population of glycoproteins. The reason for this appears to be due to variation in the carbohydrate moieties of the macromolecules. To support this contention we have used different sugar-specific lectin-sepharose affinity columns as a means for separating cobalophilins. Concanavalin A-Sepharose can resolve serum vitamin B₁₂-binding activity into three distinct fractions. The first two are cobalophilins and the third is transcobalamin II. The major cobalophilin fraction does not interact with concanavalin A-Sepharose, but is bound completely by both wheat germ and castor bean II lectins. Elution from these columns can be accomplished by addition of the appropriate competitive sugar to the buffer. Castor Bean I Lectin-Sepharose resolves the major concanavalin A-Sepharose cobalophilin fraction into two distinct fractions—one which does not interact with this lectin and the other which is eluted when N-acetylgalactosamine is added to the buffer. Furthermore, the distribution of exogenous-bound (⁵⁷Co) cyancobalamin between these two fractions is different when comparing certain myeloproliferative diseases in which serum cobalophilins are usually elevated. It is suggested that use of lectin—Sepharose resins may be a more definitive method for defining cobalophilin variants than is presently available.