Digestion of grass lipids and pigments in the sheep rumen.

I. Digestion of grass lipids and pigments in the rumen of the sheep has been studied during starvation and following the administration of "C-labelled grass. 2. Both galactolipids contained in chloroplasts are rapidly degraded, although mono-galactos~ldiglycerides disappear faster than digalactosyldiglycerides. I t was concluded that rumen microorganisms are mainly responsible for this degradation, although grass itself also contains enzymes which can degrade galactolipids. 3. Rumen contents can degrade added "C-labelled mono-and digalactosyldiglycerides in vitro at a rate sufficient to account for the disappearance of galactolipids in the intact rumen. T h e initial enzyme attack is probably a successive deacylation to give monogalactosylglycerol and digalactosylglyccrol. 4. Most of the chlorophyll pigments are rapidly converted into phaeophytins by loss of magnesium. A small proportion of chlorophyll u and more of chlorophyll b remains intact even after 24 h starvation. On the other hand, about half the phaeophytin undergoes further rapid decomposition to yield phylloerythrin. 5. rZlthough the grass phospholipids are extensively degraded, /?-carotenes and many non-polar compounds, e.g. steroids, appear to undergo little change in the rumen. It has been established that, when triglycerides are introduced into the rumen, they are rapidly hydrolysed with the liberation of free fatty acids, the glycerol also formed By comparison, the fate of the complex lipids present in plant-leaf material has received little attention. These plant-leaf lipids are found for the most part in membrane structures such as those in chloroplast lamellae, mitochondria, endoplasmic reticulum and plasma membranes, although waxes are present as constituents of the leaf cuticle. Since these membranes account for most of the dry weight of the cell mass, and they contain 20-30 yo by weight of lipids, the latter are clearly major constituents of plant cells. I n terms of mass, the main lipids of photosynthetic plants are those present in chloro-plasts, i.e. monogalactosyldiglycerides, digalactosyldiglycerides and chlorophyll pigments ; phospholipids (predominantly lecithin) are largely contained in the other membraneous structures (Kates, 1970). It can be calculated from the results of Roughan & Batt (1969) that, in a ruminant consuming perennial ryegrass (Lolium perenne L.), each g of fresh leaf tissue contains about 4 mg each of the two galactolipids and about 2.5 mg of phospholipid. This suggests a daily intake of about 4-50 g of these complex lipids in a sheep and 300-400 g in a COW consuming grass pasture. In such animals by far the major fatty acid found in …