Structural variability in the glucuronoxylomannan of Cryptococcus neoformans serotype A isolates determined by 13C NMR spectroscopy

Abstract

Cryptococcus neoformans, the etiologic agent of cryptococcal meningoencephalitis, produces glucuronoxylomannan (GXM) as the major capsule component. Purified GXMs obtained from eight serotype A isolates of C. neoformans were treated by ultrasonic irradiation and then O-deacetylated prior to their comprehensive chemical analysis by GLC, GLC-MS, and 13C NMR spectroscopy. The average xylose: mannose: glucuronic acid molar ratio of the eight isolates is 1.96 +/- 0.25: 3.00: 0.58 +/- 0.10. Methylation analyses and 13C NMR spectroscopy show a general structure for GXM that is comprised of a linear (1----3)-alpha-D-mannopyranan substituted with beta-D-GlcpA and with beta-D-Xylp at O-2. Variable quantities of unsubstituted (1----3)-alpha-D-Manp were observed between the eight isolates studied. In several isolates some of the (1----3)-alpha-D-Manp residues are disubstituted with beta-D-GlcpA at O-2 and with beta-D-Xylp at O-4; this type of substitution was not previously thought to occur in serotype A isolates. Heterogeneity, between isolates, in the disposition of the substituents along the mannopyranan backbone was revealed by 13C NMR spectroscopy. The eight isolates, and three isolates previously studied, were each assigned to one of four distinct groups based on the 13C NMR chemical shifts of the anomeric carbons. Six of the eleven isolates gave identical spectra (Group I). The six major anomeric resonances from Group I were assigned to specific glycosidic linkages present in GXM. The remaining five isolates gave more complex spectra that are indicative of additional linkages and comprise the remaining three groups. Three of these five isolates contain substantial amounts of linkages previously thought to be distinctive of serotypes B and C, i.e., Manp residues that are 4-O-glycosylated with beta-D-Xylp. Methylation analyses only predicted an average repeating unit, whereas 13C NMR spectroscopy demonstrated that GXM from each isolate may be categorized into four groups by the occurrence of distinct sequences of carbohydrate residues.