Yariv reactivity of type II arabinogalactan from larch wood.

Larch arabinogalactan (AG) is classified as a plant type II AG. Its basic structure is constituted by a β-1,3-galactan main chain with β-1,6-galactan side chains. But its properties are distinct from other type II AGs. Whereas most type II AGs are found as glycan moieties of arabinogalactan-protein (AGP), larch AG lacks a protein moiety. Larch AG itself is also unlike other type II AGs as it lacks Yariv reactivity, the capability of AG to form insoluble precipitate with β-Yariv reagents, 1,3,5-tri-(p-glycosyloxyphenylazo)-2,4,6-trihydroxybenzene with β-glucosyl or β-galactosyl residues at the termini. For the present study, we prepared β-galactan I, II, and III from larch AG by performing single, double, and triple Smith degradation, which breaks β-1,6-galactan side chains, and examined Yariv reactivity of the products. Methylation analysis revealed that β-galactans II and III had lost more than 90% of their β-1,6-galactan branches. In the radial gel diffusion assay, β-galactans II and III showed Yariv reactivity, indicating the presence of a Yariv-reactive structure in larch AG, although native larch AG does not have Yariv reactivity. The Yariv reactivity of the β-galactans was completely lost after treatment with endo-β-1,3-galactanase. These results confirm that β-1,3-galactan is necessary for Yariv reactivity of type II AG. The present results also suggest that high substitution of β-1,3-galactan with β-1,6-galactan side chains affects Yariv reactivity in larch AG.