The solubilisation and hydrolysis of spinach cell wall polysaccharides in gastric and pancreatic fluids

Abstract

Abstract:The aim of this work was to investigate the fate, in the upper gut of a monogastric animal, of polymers bound within plant cell walls. Uniformly and specifically 14C-labelled spinach cell walls were incubated in artificial body fluids with and without pepsin or pancreatin. In the absence of enzymes, artificial pancreatic juice (pH 8.8) at 37°C hydrolysed the methyl ester groups of wall-bound pectins [half-life (t1,2) 25 h]; the O-acetyl ester groups of cell wall polysaccharides were much more stable (tliz 84 h). In contrast, artificial gastric juice (pH 1.85) hydrolysed wall-bound acetyl groups more rapidly (t,,2 -24 h) than methyl ester groups ( t , / 2 -350 h). Thus, a proportion of the methyl and acetyl groups of plant cell wall polymers will be released in the upper gut as methanol and acetic acid, raising the question of whether these groups should be included within the definition of dietary fibre. The artificial body fluids also caused limited solubilisation of wall polymers but no hydrolysis to mono-or oligosaccharides. Neither pepsin nor pancreatin promoted the hydrolysis of methyl ester or acetyl groups. The small amounts of [14C]protein present in the cell wall preparations were hydrolysed by pepsin or pancreatin to yield amino acids and oligopeptides; however, the major polysaccharides of spinach cell walls were neither degraded to low-molecular-weight products nor solubilised by these enzymes.

The use of14C-labelled substrates to study plant cell wall breakdown in the gastrointestinal tract

“…Under simulated conditions of the stomach and small intestine some solubilization of labelled uronate residues from spinach CW was observed, although the solubilized material was polymeric and there was no evidence of degradation to oligogalacturonides or monosaccharides (Miller et al 1995). In agreement with this observation, only small amounts of 14C were solubilized during passage through the small intestine of the rat, but on reaching the caecum the 14C became rapidly soluble.…”

“…Artificial pancreatic juice caused the slow, non-enzymic removal of methyl ester groups (half-life 25 h) from intact CW containing [~nethyl-~~CJpectin (Miller et al 1995). However, since the CW are only present in the upper gut for 2-3 h, non-enzymic hydrolysis can only account for the release of a small proportion of the methyl ester residues in the upper gut.…”

“…The alkaline conditions of the small intestine may contribute to the partial deesterification (ester bonds are alkali-labile). However, in vitro studies have suggested that little of the loss of acetyl groups is due to the endogenous secretions of the gastrointestinal tract, as acetyl ester groups are more stable than methyl ester groups in this environment (Miller et al 1995). The most likely explanation for the observed release of acetate from CW is the initiation of degradation of CW by indigenous micro-organisms inhabiting the small intestine (Macy etal.…”

The behavior of dietary fiber in the gastrointestinal tract determines its physiological effect

“…The hydrolysis resulted in release of reducing sugars and fragments of the pectin linear chain. Miller et al (1995) also reported limited hydrolysis of ester bonds in spinach cell wall pectins, being the methyl ester groups more resistant of the O-acetyl groups at pH 1.8 whereas the opposite was observed at pH 8.8. In comparison to pectins and arabinoxylans, b-glucan is resistant to very low pH for long times (Johansson et al, 2006).…”

“…Beside structural changes, DF undergoes chemical changes in the upper intestine. By definition DF is resistant to hydrolysis by human digestive enzymes yet pancreatic lipase and other esterases might exert some hydrolytic activity toward methyl esters, O-acetyl esters, and N-acetylamide groups in DF molecules (Miller et al, 1995). At the very low pH of the stomach, the glycosidic linkages between component monosaccharides and ester linkages can suffer acid hydrolysis.…”