THE METABOLISM AND FATE OF [METHYL-C-14] AND [URONATE-6-C-14]PECTIN-LABELED DIETARY PLANT-CELL WALLS IN THE RAT

The metabolism of dietary plant cell walls (PCW) was followed in the rat using PCW isolated from spinach cell cultures which were C-14-labelled in the galacturonic acid residues (using D-[6-C-14]glucuronic acid, greater than or equal to 85% of the C-14 was confined to the C-6 position of pectic galacturonic acid residues) or the methyl ester groups (using L-[methyl-C-14]methionine, 90% of the C-14 was confined to methyl ester groups and 10% of the C-14 in methyl ether groups) of pectin. Eighteen hours after gavage, only 5-10% of the C-14 was recovered in the gut contents and faeces. Some of the C-14 derived from the [methyl-C-14]pectin-labelled PCW was present in the liver (2-4%) and the pelt (6%). In the liver the C-14 was mainly in phospholipids, whereas in the pelt the C-14 was found in protein (mainly alanine residues) and fatty acids esterified to cholesterol. The livers from rats fed [uronate-6-C-14]pectin-labelled PCW contained less C-14 (1%) but more C-14 was found in the pelt (12-15%). In the liver the C-14 was found in phospholipid but in the pelt the label was exclusively found in amino acid residues. Pectin in intact PCW is extensively degraded in the caecum and colon and the products absorbed and metabolised by the host. These products may be important sources of energy and of precursors of structural compounds such as proteins and lipids.