Abstract
The galactomannan of surface grown Aspergillus niger has been isolated by alkaline extraction of hyphal walls and characterized structurally. Its elution profile, from a column of Bio-Gel P-150, reveals a broad range of molecular sizes grouped into two fractions. Gas chromatographic and colorimetric analyses indicate that each fraction is composed of approximately equimolar quantities of galactose and mannose plus 12 to 14% glucose. Both have similar low optical rotations and contain acid labile galactose. Methylation, Smith degradation, acetolysis, reactivity with concanavalin A and beta-D-galactofuranosidase, plus digestionof galactose with D-galactose oxidase, were techniques employed to determine the polysaccharide's covalent structure. Results of these studies indicate that it is composed of a series of chains, 5 to 9 hexose units in length, connected by alpha1 leads to 6 bonds between mannopyranosyl moieties. The external portion of each chain consists of a galactose tri- or tetrasaccharide of the general structure Galf beta1 leads to 4 Galp(1-2) 1 leads to 4 Galp1 leads to. This segment is connected via a (1 leads to 2) linkage to the internal portion which is a di- to pentasaccharide of mannopyranosyl units joined in alpha1 leads to 2 glycoside linkage. Combination mild acid hydrolysis and methylation experiments indicate galactofuranosyl terminal units are attached only to galactose. The organization of glucose into the overall structure of the polymer has not been determined. Structural relationships of this polysaccharide to both fungal and yeast galactomannans are discussed.
Highlights
The galactomannan of surface grown Aspergillus niger has been isolated by alkaline extraction of hyphal walls and characterized structurally
Because of the small amount of glucose observed on the chromatograms and the fact that a glucan co-fractionated with the galactomannan through much of the purification procedure, gel filtration chromatography was utilized (Fig. 1) to see if additional glucan would be separated from the polysaccharide by this method and to monitor the ratio of galactose to mannose across the elution profile
VI was noted a small (10%) drop could not be ruled out at the level of sensitivity of this experiment. These results strongly indicate that essentially all terminal galactofuranosyl units are bonded to gala&se and not mannose and some of the gala&se occupying internal positions in the polymer is in the pyranose form
Summary
A and yeast mannan were purchased from Sigma, galactose oxidase from Worthington Biochemicals and carbohydrates from Pfanstiehl. Mycodextranase was prepared as described previously [17]. OV-210 coated on Supelcoport (80 to 100 mesh) was obtained from Supelco Inc. and ECNNS-M and Gas-. Precoated thin layer cellulose plates (250 pm) were purchased from Brinkmann Instruments and Bio-Gel acrylamide resins from Bio-Rad Inc. Methylated mannitol acetate standards for gas chromatography were a gift from. E. Ballou of the University of California, Berkeley. Growth ofAspergillus niger NRRL 326 -The organism was grown in surface culture from spore inoculum at 20-22” in the growth medium described previously [16] using Pyrex dishes (34 x 20 x 4 cm) each containing 175 ml of medium.
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