Hydrolysis of triacylglycerols

FIGURE 24.3 (a) A duct at the junction of the pancreas and duodenum secretes pancreatic juice into the duodenum, the first portion of the small intestine, (b) Hydrolysis of triacylglycerols by pancreatic and intestinal lipases. Pancreatic lipases cleave fatty acids at the C-1 and C-3 positions. Resulting monoacylglycerols with fatty acids at C-2 are hydrolyzed by intestinal lipases. Fatty acids and monoacylglycerols are absorbed through the intestinal wall and assembled into lipoprotein aggregates termed chylomicrons (discussed in Chapter 25). 

Increased lipid synthesis/inhibi-tion of lipolysis Activation of lipoprotein lipase (LPL)/induc-tion of fatty acid synthase (FAS)/inactivation of hormone sensitive lipase (HSL) Facilitated uptake of fatty acids by LPL-dependent hydrolysis of triacylglycerol from circulating lipoproteins. Increased lipid synthesis through Akt-mediated FAS-expression. Inhibition of lipolysis by preventing cAMP-dependent activation of HSL (insulin-dependent activation of phosphodiesterases )... 

The triacylglycerols (Figure 14—6) are esters of the tri-hydric alcohol glycerol and fatty acids. Mono- and di-acylglycerols wherein one or two fatty acids are esteri-fied with glycerol are also found in the tissues. These are of particular significance in the synthesis and hydrolysis of triacylglycerols. 

The hydrolysis of triacylglycerol is catalysed by lipases, two of which are present in the stomach. These are lingual lipase, which is secreted by the soft palate, and gastric lipase, which is secreted by the gastric glands of the stomach. Gastric lipase is particnlarly important in the newborn since, at this stage of life, pancreatic secretions contain relatively little lipase. 

Figure 7.3 The action of lipoprotein lipase in the hydrolysis of triacylglycerol in the blood and the fate of the fatty adds produced. Lipoprotein Lipase is attached to the luminal surface of the capillaries in the tissues that are responsible for removal of triacylglycerol from the bloodstream (e.g. adipose tissue, muscle, lactating mammary gland).

Most of the long-chain fatty acids that are oxidised in the body are released from triacylglycerol in adipose tissue. The first step is hydrolysis of triacylglycerol within the... 

The physiological pathway for oxidation of ketone bodies starts with the hydrolysis of triacylglycerol in adipose tissue, which provides fatty acids that are taken up by the liver, oxidised to acetyl-CoAby P-oxidation and the acetyl-CoA is converted to ketone bodies, via the synthetic part of the pathway. Both hydroxybutyrate and acetoacetate are taken up by the tissues, which can oxidise them to generate ATP (Figure 7.19). 

Hydrolysis of triacylglycerol and release of the resultant fatty acids from adipose tissue is increased and accounts, in part, for loss of body weight. Despite this, the plasma fatty acid level is not always increased, which suggests... 

Hydrolysis of triacylglycerols is catalyzed by lipoprotein lipase, a membrane-bound enzyme located on the endothelium lining the capillary beds of the muscle and adipose tissue. 

Fate of free fatty acids The free fatty acids derived from hydrolysis of triacylglycerol may directly enter adjacent mus cells or adipocytes. Alternatively, the free fatty acids may be tra ported in the blood in association with serum albumin until tt are taken up by cells. [Note Serum albumin is a large prot secreted by the liver. It transports a number of primarily hydropl bic compounds in the circulation, including free fatty acids a some drugs.2] Most cells can oxidize fatty acids to produ energy (see p. f88). Adipocytes can also reesterify free fa acids to produce triacylglycerol molecules, which are stored ui the fatty acids are needed by the body (see p. 185). 

Chylomicrons transport dietary triacylglycerol and cholesteryl ester from the intestine to other tissues in the body. Very-low-density lipoprotein functions in a manner similar to the transport of endogenously made lipid from the liver to other tissues. These two types of triacylglycerol-rich particles are initially degraded by the action of lipoprotein lipase, an extracellular enzyme that is most active within the capillaries of adipose tissue, cardiac and skeletal muscle, and the lactating mammary gland. Lipoprotein lipase catalyzes the hydrolysis of triacylglycerols (see fig. 18.3). The enzyme is specifically activated by apoprotein C-II, which... 

DAG species are derived from three main routes (1) PLC-mediated hydrolysis of phospholipids (2) phosphatase-mediated hydrolysis of phosphatidic acid (PA) and (3) lipase-mediated hydrolysis of triacylglycerol (TAG) species (Fig. 2). Targeted lipidomic analyses show that the fatty acid compositions of the DAGs formed by these various routes reflect the composition of the parent lipid (Fig. 5). In particular, those derived from inositol phospholipids are highly enriched in... 

The hydrolysis of triacylglycerol to monoacylglycerol and fatty acids by hormone-sensitive lipase can be stimulated by epinephrine, norepinephrine, adrenal steroids, glucagon, and the hypophysial hormones, luteotropin (prolactin or luteinizing hormone), )3- and a-lipotropins, somatotropin, thyrotropin, and vasopressin. 

The release of fatty acids from adipose tissue is regulated by the rate of hydrolysis of triacylglycerol and the rate of esterification of acyl-CoA with glycerol 3-phosphate. The rate of hydrolysis is stimulated by hormones that bind to cell-surface receptors and stimulate adenylate cyclase (which catalyzes the production of cAMP from ATP). Hormone-sensitive lipase (Sec. 13.4) can exist in two forms, one of which exhibits very low activity and a second which is phosphorylated and has high activity. Before hormonal stimulation of adenylate cyclase, the low-activity lipase predominates in the fat cell. Stimulation of protein kinase by an increase in cAMP concentration leads to phosphorylation of the low-activity lipase. An increase in the rate of hydrolysis of triacylglycerol and the release of fatty acids from the fat cell follows. This leads to a greater utilization of fatty acids by tissues such as heart, skeletal muscle, and liver. 

LPL catalyses the hydrolysis of triacylglycerols to hherate free fatty acids and glycerol. LPL is specifically found in endothelial cells lining the capillaries. LPL has different isoenzyme forms in different tissues that form found in adipocytes is activated hy insuhn (which helps to explain why adipose cells gain fat in a well-fed state). 

Figure 12.8. The synthesis and hydrolysis of triacylglycerols are catalyzed by lipases.

The initial event in the utilization of fat as an energy source is the hydrolysis of triacylglycerols by lipases, an event referred to as lipolysis. The lipase of adipose tissue are activated on treatment of these cells with the hormones epinephrine, norepinephrine, glucagon, and adrenocorticotropic hormone. In adipose cells, these hormones trigger 7TM receptors that activate adenylate cyclase (Section 15,1.3 ). The increased level of cyclic AMP then stimulates protein kinase A, -which activates the lipases by phosphorylating them. Thus, epinephrine, norepinephrine, glucagon, and adrenocorticotropic hormone induce lipolysis (Figure 22.6). In contrast, insulin inhibits lipolysis. The released fatty acids are not soluble in blood plasma, and so, on release, serum albumin binds the fatty acids and serves as a carrier. By these means, free fatty acids are made accessible as a fuel in other tissues. 

Many students have received degrees in recent years for exploring enrichment of marine oils (or other oils) by selective hydrolysis of triacylglycerols, or ester interchanges between esters and natural oils by enzymes. These explorations tend to be somewhat theoretical (100), but they can be effective, although impractical, for example, a 100-hour reaction time (101). Having a starting material rich in the desired product fatty acid (DHA) helped in one case (102), but the complexity of these proposed processes requires a separate article. 

In all species, the small intestine is the main site for simultaneous hydrolysis of fats, proteins, and carbohydrates by selective enzymes and absorption of the resulting nutrients. It consists of three sequential sections duodenum, jejunum and ileum, each with villi and mucosal linings. During the process, the pH of the digesta is raised from that of the stomach, to near neutrality over the length of the small intestine. In swine, the pH profile is as follows stomach, 2.4 proximal duodenum, 6.1 distal duodenum, 6.8 proximal jejunum, 7.4 distal jejunum, 7.4 and ileum, 7.5. In sheep, the profile is abomasum, 2.0 proximal duodenum, 2.5 distal duodenum, 3.5 proximal jejunum, 3.6 distal jejunum, 4.7 and ileum, 8.0 (48). Several types of contractive and peristaltic actions mix and move the digesta down the intestine. The lower pH at the proximal duodenum of ruminants plays a critical part in fatty acid reabsorption. Hydrolysis of triacylglycerols by pancreatic lipase... 

Hydrolysis of triacylglycerol may be accomplished enzymatically through the action of lipases. 

Lipolysis hydrolysis of triacylglycerol to glycerol and free fatty acids... 

So far, only very little attention has been focussed on the use of zeolites in biocatalysis, i.e., as supports for the immobilization of enzymes. Lie and Molin [116] studied the influence of hydrophobicity (dealuminated mordenite) and hydrophilicity (zeolite NaY) of the support on the adsorption of lipase from Candida cylindracea. The adsorption was achieved by precipitation of the enzyme with acetone. Hydrolysis of triacylglycerols and esterification of fatty acids with glycerol were the reactions studied. It was observed that the nature of the zeolite support has a significant influence on enzyme catalysis. Hydrolysis was blocked on the hydrophobic mordenite, but the esterification reaction was mediated. This reaction was, on the other hand, almost completely suppressed on the hydrophilic faujasite. The adsorption of enzymes on supports was also intensively examined with alkaline phosphatase on bentolite-L clay. The pH of the solution turned out to be very important both for the immobilization and for the activity of the enzyme [117]. Acid phosphatase from potato was immobilized onto zeolite NaX [118]. Also in this study, adsorption conditions were important in causing even multilayer formation of the enzyme on the zeolite. The influence of the cations in the zeolite support was scrutinized as well, and zeolite NaX turned out to be a better adsorbent than LiX orKX. 

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