Animal TAG lipases may be broadly classified into extra-cellular and intracellular. Extra-cellular lipa-ses are secreted from their cells of synthesis and act upon TAG that are present in the extra-cellular environment, releasing fatty acids that may be taken up into cells. Therefore, they are concerned mainly with cellular uptake of fatty acids. There are four major extra-cellular TAG lipases (in adults), which are members of one family and have sequence and structural similarities to each other. Pancreatic lipase is, as its name suggests, secreted from the exocrine tissue of the pancreas into the small intestine, where it acts to hydrolyse dietary TAG so that the constituent fatty acids and mono-acylglycerols may be absorbed from the intestine into enterocytes. Its action is described in more detail in Section 5.1.1.
Within the body, TAG circulates in the plasma in the form of macromolecular complexes, the lipo-proteins (Section 5.2). Some lipoprotein TAG is taken up directly into cells that express receptors that bind and internalize complete particles, but this is mainly a route for the clearance of 'remnant' particles, once they have lost most of their TAG. Most of the TAG is hydrolysed within the vascular compartment by the enzyme lipoprotein lipase, to release fatty acids that may again be taken up into cells. (This enzyme is called clearing factor lipase in older literature, because of its ability to clear the turbidity of the plasma caused by the presence of large TAG-rich lipoprotein particles.) Lipoprotein lipase is related to pancreatic lipase. It is synthesized within the parenchymal cells of tissues (e.g. adipocytes, muscle cells, milk-producing cells of the mammary gland), but then exported to the endothelial cells that line blood capillaries. Here it is bound to the luminal membrane of the endo-thelial cell (facing into the blood) where it can act upon the TAG in lipoprotein particles as they pass by. The fatty acids it releases may then diffuse into the adjacent cells, for re-esterification and storage (e.g. in adipose tissue) or for oxidation (in muscle).
Hepatic lipase is the third member of this family. As its name suggests, it is synthesized within the liver and, just like lipoprotein lipase, exported to bind to the endothelial cells that line the hepatic sinusoids - the tiny vessels that are the liver's equivalent of capillaries. It plays a role particularly in hydrolysis of TAG in smaller lipoprotein particles, and also assists in binding these particles to the receptors that may remove them from the circulation.
The fourth member of the family is known as endothelial lipase. It was discovered only in 1999 and its function is not yet clear. It is expressed by the endothelial cells in several tissues including liver, lung, kidney and placenta, and some endocrine tissues including thyroid, ovary and testis. It is more active as a phospholipase (with A1-type activity, Section 7.2) than as a TAG lipase. It has been suggested that it plays a role in lipoprotein metabolism and vascular biology.
Of the intracellular lipases, one has been particularly well studied: the so-called hormone-sensitive lipase expressed in adipocytes and in some cells with an active pattern of steroid metabolism (adrenal cortex cells, macrophages, testis). Its name reflects the fact that its activity is rapidly regulated by a number of hormones (Section 3.6.3) although this can be confusing because other lipases, especially lipoprotein lipase, are also regulated by hormones. Hormone-sensitive lipase acts on the surface of the triacylglycerol droplet stored within adipocytes, to release fatty acids that may be delivered into the circulation for transport to other tissues where they may be substrates for oxidation or for re-esterification to glycerol. Hormone-sensitive lipase is also a cholesteryl esterase and acts with equal efficiency to hydrolyse cholesteryl esters: this is presumably its role in cells that metabolize steroids. Adipose tissue also contains a lipase that is much more active on mono-acylglycerols than is hormone-sensitive lipase.
Lipases are still being discovered and it may be a long time before the whole complex jigsaw puzzle of glyceride breakdown can be pieced together. For instance, it is known that an intracellular lipase is involved in hydrolysis of triacylglycerol within hepatocytes, releasing fatty acids that are then re-esterified before incorporation into very low density lipoprotein particles (the lipoprotein particles secreted by the liver; see Section 5.2.3). This lipase has not yet been characterized.
3.5.3 Plant lipases break down the lipids stored in seeds in a specialized organelle, the glyoxysome
Seeds that contain lipid may have as much as 80% of their dry weight represented by triacylglycerols. Plants such as soybean face two particular problems in using such energy reserves. First, these plants have to mobilize the lipid rapidly and break it down to useful products. This overall process involves the synthesis of degradative enzymes as well as the production of the necessary membranes and organelles that are the sites of such catabolism. Secondly, plants with lipid-rich seeds must be able to form water-soluble carbohydrates (mainly sucrose) from the lipid as a supply of carbon to the rapidly elongating stems and roots. Animals are unable to convert lipid into carbohydrate (Fig. 3.9) because of the decarboxylation reactions of the Krebs (tricarboxylic acid) cycle (isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase). Thus, for every two carbons entering the Krebs cycle from lipid as acetyl-CoA, two carbons are lost as C02. In plants, these decarboxylations are avoided by a modification of the Krebs cycle, which is called the glyoxylate cycle. This allows lipid carbon to contribute to the synthesis of oxaloacetate, which is an effective precursor of glucose and hence, sucrose.
When water is imbibed into a dry seed, there is a sudden activation of metabolism once the total water content has reached a certain critical proportion. So far as lipid-storing seeds are concerned, TAG lipase activity is induced and studies by Huang in California and others suggest that this lipase interacts with the outside of the oil droplet through a specific protein (an oleosin; Section 3.3.3) that aids its binding. This interaction has some similarities to the binding of lipoprotein lipase to very low density lipoproteins in animals (Sections 3.5.2 and 5.2.5). The lipases in seeds hydrolyse the 1 and 3 positions of TAG and because the acyl group of the 2-monoacylglycerol products can migrate rapidly to position 1, the lipases can completely degrade the lipid stores.
The fatty acids that are liberated are activated to CoA-esters and broken down by a modification of (3-oxidation, which takes place in specialized microbodies. Because these microbodies also contain the enzymes of the glyoxylate cycle (see above), they have been termed glyoxysomes by the Cali-fornian biochemist, Harry Beevers. Beevers and his group worked out methods for the isolation of
glyoxysomes from germinating castor bean seeds and showed by careful study that fatty acid P-oxi-dation was confined to this organelle. The glyoxysomes have only a temporary existence. They are formed during the first two days of germination and, once the lipid stores have disappeared (after about 6 days), they gradually break down. Nevertheless, in leaf tissues, the role of glyoxysomes in the P-oxidation of fatty acids is replaced by other microbodies (Section 22.214.171.124).
Most plant lipases appear to be membrane-bound although soluble enzymes are present in some tissues. There has been some interest in using the latter in the food industry in order to carry out the transesterification of triacylglycerols and, thus, modify their composition for certain purposes [e.g. to make fats that resemble cocoa butter ('cocoa butter equivalents') for chocolate]. These transes-terification reactions are favoured by very low water contents (usually less than 1%).
Plant lipase activities also have implications for food quality, when the release of unesterified fatty acids can cause deterioration. For example, during olive oil production, significant endogenous fatty acid levels, themselves, determine whether an oil can be considered high quality ('extra virgin') or not. Alternatively, in poorly-stored wholemeal flour, endogenous lipases give rise to fatty acids that can be broken down by lipoxygenases (Sections 2.3.5 and 2.3.6) and cause off-flavours and a reduction in baking quality.
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