Arabinogalactan

Arabinogalactans have been isolated from the tissues of a variety of dicots. However, no arabinogalactan has been isolated from a source known to contain only primary cell walls. The glycosyl compositions of the arabinogalactans isolated from rapeseed cotyledons, rapeseed flour, larch wood, maple sap, the medium of suspension-cultured tobacco cells, the medium of suspension-cultured sycamore cells, and from soybean cotyledons are summarized in Table 2. 

The arabinogalactans are also related to one another in that they all appear to have a galactan backbone with arabinosyl sidechains. The structures of these polysaccharides have been further investigated by partial acid hydrolysis and by periodate oxidation 14, 72, 87, 116). The arabinosyl residues are hydrolyzed preferentially by mild acidic 

Glycosyl Rapeseed Rapeseed Maple Extracellular Extracellular Extracellular Soybean 

Linkage cotyledon flour Larch sap Tobacco Sycamore I Sycamore II cotyledon 

The xylosyl residues have been left out of this calculation (see Table 2). Approximate linkage quantitation is represented by + through + + + +. Contains 7% T-GlcA. 

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Larch Gum. Larch gum [37320-79-9] (larch arabinogalactan) is obtained by water extraction of the western larch tree, iLarix occidentalism the heartwood of which contains 5—35% on a dry wood basis. In the early 1960s, a countercurrent hot water extraction system was developed, and the gum was produced commercially by the St. Regis Paper Co. under the trade name Stractan. The potential production capacity of this gum is 10,000 t/yr based on the wood residues from the lumber industry. However, the product could not compete with gum arabic, and commercial production is now limited to small batches for a specific medical appHcation. 

StmcturaHy, arabinogalactan is a complex, highly branched polymer of arabinose and galactose in a 1 6 ratio (67). It is composed of one fraction with an average molecular weight of 16,000, and one of 100,000 (68). 

Larch arabinogalactan is approved in 21 CFR 172.610 as a food additive for use as an emulsifier, stabilizer, binder or bodying agent for essential oils and noimutritive sweeteners, flavor bases, nonstandardized dressings, and pudding mixes. It has also been used in the preparation of cosmetic and pharmaceutical dispersions and as an emulsifier in oil—water emulsions (69). Industrially, the main use has been in Hthography as a gum arabic substitute. 

The common hemiceUulose components of arborescent plants are listed in Table 3. Xylans, arabinogalactans, and pectic substances are common to all while only traces (if at all) of glucomaimans are found in the cell walls of bamboo. Other polysaccharides are found in trace amounts in wood as well as in bark, growing tissues, and other specialized parts of trees. 

Within the scope of this review, the contributions of the last decade concerning cell-wall polysaccharides isolated from woody and other plant tissues will be reviewed according to the above-proposed classification of hemicelluloses including larch arabinogalactans. The present review article updates and extends previous reviews [3-5] and will focus in particular on new investigated plant sources, isolation methods, structural features, physicochemical and various functional properties of hemicelluloses. Attention will also be paid to the modification of isolated hemicelluloses or hemicellulosic materials and the appHcation possibiUties of hemicelluloses and their derivatives, including their use for the production of composite materials and other biomaterials. 

Arabinogalactans (AGs) are widely spread throughout the plant kingdom. Many edible and inedible plants are rich sources of these polysaccharides. AGs occur in two structurally different forms described as type I and type II, associated with the pectin cell-wall component by physical bonds and some of them are covalently linked to the complex pectin molecule as neutral side chains. Commercial pectins always contain AG 10-15%). AG of type I has a linear (1 4)-y0-o-Galp backbone, bearing 20-40% of of-L-Ara/ residues (1 5)-linked in short chains, in general at position 3. It is commonly found in pectins from citrus, apple and potato [6]. Recently, this AG type has been isolated from the skin of Opuntia ficus indica pear fruits [372]. 

Keywords Arabinogalactans Bioactivity Medicinal plants Rhamnogalacturonans Pectins Structure-activity relations... 

The arabinogalactans have more frequently been reported for activity in various biological systems. Arabinogalactans are often classified in three groups arabino-4-galactans (Type 1), arabino-3,6-galactans (Type II) and polysaccharides with arabinogalactan side chains (Type III) [14]. The latter type are also called the real pectins [10,11]. Only types I and II will be dealt with in this chapter, as Type III are equal to the pectins discussed below. 

Fig. 2 Average structure of the hairy or ramified region of an apectic substance, with a rhamnogalacturonan I backbone substituted at position 4 of the rhamnose units with arabinan and arabinogalactan type II side chains...

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