GumArabic is a Kind of Arabinogalactan-Protein 1

generally recognized as an acidic polysaccharide having a core structure of highly branched, 1,3; l,6-galactan,5) it has been reported that it also contains nitrogen (i.e., protein)6) and that it is precipitated with artificial carbohydrate antigen (Yariv antigen),7} which is one of the properties of AGPs. In fact, gum arabic reacted with /?galactosyl Yariv antigen in gel-diffusion experiments3} just like tobacco AGPs, and the rabbit antiserum to crude AGP from tobacco ECP reacted with gum arabic (S. Yamamoto,private communication). The above facts suggested that the gum arabic is a kind of AGP. Table I shows the results of chemical composition and methylation analyses of commercially available gum arabic (purchased from Kanto Chemical, Tokyo, Test No. 51 1A3042), which was purified using DEAE-Sephadex A25. As shown in Table I, gum arabic consists of neutral sugars, uronic acid, protein and hexosamine. The amino acid composition of gum arabic is shown in Table II, column A. It is rich in hydroxyproline and serine, but the alanine content is low. Since gum arabic was thought to be a kind ofAGP, its carbohydrate-protein linkage was investigated. Gumarabic was hydrolysed with saturated barium hydroxide8} to examine the hydroxyproline-glycoside pattern. The hydrolysate was fractionated on a column of Bio-Gel P-2 as shown in Fig. 1. About 23% of the hydroxyproline appeared at void volume fraction. The material in this fraction was found to be similar to the original gum arabic in sugar composition and glycosidic linkage from the results of sugar and methylation analyses. About 13% of Table I. Chemical Composition and Methylation Analyses of GumArabic0