CHARACTERIZATION OF GUM-ARABIC FRACTIONS OBTAINED BY ANION-EXCHANGE CHROMATOGRAPHY

Two samples of authentic gum arabic (A and B) have been fractionated by anion-exchange chromatography on DEAE-cellulose, The fractions were isolated by a step-wise increase in the ionic strength of the elution buffer. Samples A and B yielded five and six fractions, respectively. All of the fractions isolated were polydisperse containing varying proportions of the different M(r) species associated with the whole gum. The carbohydrate composition of all fractions remained relatively constant with each containing similar proportions of galactose, arabinose, rhamnose and glucuronic acid. The protein content of the fractions varied slightly (0.31-2.8%). For sample A, the protein content decreased in the order F2 > F1 > F3 > F5 > F4; for sample B the order was F1 > F6 > F3 > F2 > F4 > F5. Whereas Hyp and Ser were the principal amino acids for the whole gums and the majority of the fractions isolated, two of the fractions from sample A had Asp, Ser and Glu as their principal components. Interaction of the fractions from both samples with an artificial carbohydrate antigen (Yariv reagent) indicated that they all contained arabinogalactan proteins (AGP). In addition, immune-dot blots of these fractions screened against a panel of anti-AGP monoclonal antibodies demonstrated that they all contained epitopes recognized by one or more of these antibodies. These data describe the results of the first chemical characterization of the molecular components of gum arabic fractionated using anion-exchange chromatography. They further demonstrate the molecular complexity of this widely studied natural product, and in combination with other separation procedures may provide the key to the eventual elucidation of the biochemical synthesis of this fascinating and useful substance.