Sources of Pglucans

Six types of P-glucans have been identified in the cell walls of green plants and fungi. Cellulose is a (1 — 4)-P-glucan, a long linear glucose polymer, with low flexibility and solubility. It is a ubiquitous component of the fibril-lar phase of cell walls while it is also found as an extracellular secretion in certain bacteria.2

In cereals, cellulose microfibrils appear to be embedded in a matrix of mixed-linkage (1 — 3, 1 — 4)-P-glucan in the walls of aleurone and starchy endosperm cells. Oat (Avena sativa) and barley (Hordeum vulgare) both contain large amounts of (1 — 3,1 — 4)-P-glucan, the latter constituting approximately 75% of the walls of their starchy endosperm cells. The P-glucan found in oat and barley is a soluble fibre that is easily fermented in the intestine and that has a high nutritive value. In addition, the endosperm cell walls of other monocots of the grass family Gramineae - such as wheat (Triticum aestivum), rye (Secale cereale) and rice (Oryza sativa) - contain small amounts of P-glucans.3'4

A range of fluorescent dyes have been studied for their interaction with endosperm cell walls showing that Calcofluor and Congo red can be used as sensitive and specific markers for the detection of P-glucan.5 Calcofluor binding reveals that the distribution of P-glucan in the kernels of barley is generally even whereas the concentration of P-glucan is particularly high in the subaleurone layer of the oat groat and this may contribute significantly to the water-binding capacity of the bran and to its efficacy as dietary fibre. Hence dry milling and air-sifting techniques have been exploited to manufacture enriched fractions of oats.67 Enrichment methods have also been employed for barley because the grain is to some extent morphologically differentiated.8,9 Plant breeding has developed oat and barley cultivars with varying contents of P-glucan.10 Evaluation of groat characteristics and groat composition in 35 genotypes from 9 taxonomic species of Avena has revealed a variation in P-glucan from 2.2 to H.3%.11

Barley has traditionally been seen as a food for human consumption. On the other hand, oat has gained extra attention due to a large number of clinical studies that identified oat P-glucan as the component responsible for the significant cholesterol-lowering properties attributed to oats, particularly the decrease of low-density lipoprotein (LDL) cholesterol.12 Oat P-glucan hydrolysates are furthermore thought to possess a probiotic effect.13 Higher plants not only contain significant amounts of mixed linkage (1 — 3, 1 — 4)-p-glucan but also (1 — 3)-P-glucan with a single monomer and linkage type. Deposits of (1 — 3)-P-glucans, referred to as callose, occur in the cell walls of plant tissues during their development and are found in specialized cell walls in response to wounding, infection and physiological stress.14 Certain bacteria secrete extracellularly a polymer of similar structure, namely curdlan.

Microbial P-glucan is a component of cell walls or is secreted by microorganisms in the growing medium, as in the case of lactic acid bacteria that are living and growing on plants, often under harsh conditions, such as Pediococcus damnosus.15 The glucose molecules in these polymers are generally connected with (1 — 3) linkages with varying proportions of (1 — 6)-

linked P-glucosyl residues substituted at intervals along, or as branches of, the (1 — 3) backbone.

Mushroom myco-polysaccharides such as P-glucan lentinan from shiitake Lentinus edodes are also comprised of a P-(1 — 3)-D-glucan backbone with P-(1 — 6)-glucan side chains. The content of P-glucan is typically around 0.3 g per 100 g of mushroom on a dry basis whereas its distribution in the soluble fraction of total dietary fibre ranges widely from 17 to 46%. The soluble: insoluble ratio varies depending on the particular mushroom whereas the content of soluble P-glucan is higher in cereals.16

Yeast P-D-glucan, also a polyglucose polysaccharide, derived from the cell walls of baker's yeast or Saccharomyces cerevisiae, consists of straight-chain and branched polymers. The straight-chain structures are (1 — 3)-P-D-linked glucose polymers and (1 — 6)-P-D-linked glucose polymers. Similarly the homopolysaccharide of glucose produced by Botryosphaeria rhodina (laminaran) consists of a (1 — 3)-P-D-linked backbone containing varying degrees of (1 — 6)-P branches.17 Yeast P-glucan has been demonstrated to have non-specific immune-enhancing effects in in vitro and some animal and human studies.18

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