Digestion and absorption of galactolipids

Abstract: Galactolipids: digalactosyldiglyceride (DGalDG), monogalactosyldiglyceride (MGalDG) and sulfoquinovosyldiglyceride (SQDG) are the major lipid components in thylakoid membranes of chloroplasts i.e. the site of photosynthesis in plants. They are the most abundant membrane lipids in nature, and thus important sources of essential fatty acids for herbivores and omnivores. This thesis investigates the digestion and absorption of galactolipids in the gastrointestinal tract. Human pancreatic juice and duodenal content was found to hydrolyse galactolipids. The carboxylester lipase (CEL) had galactolipase activity whereas the colipase dependent lipase did not. Neither of these two enzymes could however, account for the efficient hydrolysis of galactolipids by crude pancreatic juice. Pancreatic lipase related protein 2 from guinea pig and rat displayed hydrolytic activities towards galactolipids. The galactolipase activity of guinea pig PLRP2 was higher than that of rat PLRP2. We purified human PLRP2 from pancreatic juice, which possessed galactolipase activity at similar level as the rat PLRP2. The conclusion was that both PLRP2 and CEL contribute to the digestion of galactolipids. In vivo, tritium-fatty acid labeled DGalDG was given to mesenteric lymph duct cannulated or intact rats. No DGalDG was found in chyle and radioactivity occurred mainly in triglycerides. Little or no intact DGalDG was found in plasma or liver. Intravenously injected radiolabeled galactolipid liposomes were rapidly cleared from plasma in rats and readily taken up and degraded by the hepatocytes of the liver. The conclusion was that little or no dietary galactolipids enter the blood intact. If minor amounts do enter blood the liver has a good capacity to degrade them.