Lipolysis enzymic hydrolysis

Lipolysis, the enzymic hydrolysis of milk lipids to free fatty acids and partial glycerides, is a constant concern to the dairy industry because of the detrimental effects it can have on the flavor and other properties of milk and milk products. However, free fatty acids also contribute to the desirable flavor of milk and milk products when present at low concentrations and, in some cheeses, when present at high concentrations. 

Downey (1980) reasoned that although milk lipoprotein lipase is present in sufficient amounts to cause extensive hydrolysis and potential marked flavor impairment, this does not happen in practice for the following reasons (1) the fat globule membrane separates the milk fat from the enzyme, whose activity is further diminished by (2) its occlusion by casein micelles (Downey and Murphy 1975) and by (3) the possible presence in milk of inhibitors of lipolysis (Deeth and Fitz-Gerald 1975). The presence in milk of activators and their relative concentration may also determine whether milk will be spontaneously rancid or not (Jellema 1975 Driessen and Stadhouders 1974A Murphy et al. 1979 Anderson 1979). 

Phospholipids also have a role in the LPL-catalyzed hydrolysis of triglycerides. The activator apo-LPs exhibit enhanced activation in the presence of phospholipids such as phosphatidyl choline (La Rosa et al., 1970 Blaton et al., 1974) and in milk there is evidence that apo-LPs in the absence of phospholipids are unable to initiate lipolysis of intact milk fat globules by the indigenous LPL (Driessen and Stadhouders, 1974 Clegg, 1980). The phospholipids are believed to be involved in the reaction through their interaction with the substrate rather than with the enzyme (Blaton et al., 1974). 

Figure 26-5. Principle of the 13C-mixed triglyceride breath test. Absorption of 13C-mixed triglycerides requires prior hydrolysis by pancreatic lipase (1), which leads to production of free fatty acids (stearic acid) and monoacylglycerol [2-(l-13C)octanoylglycerol]. These metabolites are incorporated into micelles, absorbed, and transported to the liver (2). Further degradation by hepatic enzymes and P-oxidation results in formation of 13C02, which is absorbed into the bloodstream, transported to the lung, and exhaled (3). Thus, exhalation of 13C02 reflects intestinal lipolysis and is a marker of pancreatic exocrine function.

Moreover DBcAMP did not inhibit the hydrolysis of cAMP by heart phosphodiesterase, indicating a failure to bind to the enzyme. Since DBcAMP is not degraded by the diesterase its biological activity should not be influenced by those materials which either inhibit (theophylline) or stimulate (insulin, nicotinic acid, imidazole) the enzyme. The effect of these compounds on llpolysls produced by DBcAMP has been studied with somewhat conflicting results. Theophylline was found to potentiate DBcAMP-Induced lipolysis24,79 and nicotinic acid inhibited DBcAMP-induced lipo-lysls27,79. Insulin and imidazole have been reported to have either no effect or to inhibit lipolysis induced by DBcAMP. These results are difficult to interpret at this time. 

Partial hydrolysis of lipids occur during digestion under the influence of lipases (Sections 11.2 and 11.3). Lipases present in seeds also promote hydrolysis, so that most extracted lipids contain some free acid and some partial glycerides. This is an undesirable change, since removal of free acid during processing is accompanied by some loss of fat. Lipolysis can be minimized by inactivation of the lipase before extraction. Enzymic deacylation is also the basis of some valuable analytical techniques (Section 6.2.17). 

Such a hydrolysis is termed saponification since it produces soaps, which are sodium and potassium salts of the fatty acids. The process of fat breakdown may take place naturally xmder the influence of enzymes, collectively known as lipases, when it is termed lipolysis.The enzymes may have a certain specificity and preferentially catalyse hydrolysis at particular positions in the molecule. Removal of the fatty acid residue attached to carbon atom 2 of an acylglycerol is more difficult than those at positions 1 and 3. Under natural conditions, the products of lipolysis are usually mixtures of mono- and diacylglycerols with free fatty acids. Most of these acids are odourless and tasteless, but some of the lower ones, particularly butyric and caproic, have extremely powerful tastes and smells when such a breakdown takes place in an edible fat, it may frequently be rendered completely imacceptable to the consumer. The lipases are mostly derived from bacteria and moulds, which are chiefly responsible for this type of spoilage, commonly referred to as randdity. Extensive lipolysis of dietary fats takes place in the duodenum and during their absorption from the small intestine. Lipolysis also precedes the hydrogenation of fats in the rumen, and the oxidation of fats in the body. 

Hydrolysis of esterified lipids is essential in order for lipid absorption to occur. Anatomically, lipolysis begins in the stomach where a limited amount of partial hydrolysis of triglycerides provides more polar lipids (fatty acids and partial glycerides) to assist in emulsification. A lingual lipase is thought to be involved in this process. The net result of this gastric phase of fat digestion is the initiation of lipolysis and the preparation of a stable emulsion on which the major pancreatic lipolytic enzymes can act in the milieu of the small intestine. 

In the dairy industry, lipases are used in the hydrolysis of milk fat. Applications include flavor enhancement of cheeses, acceleration of cheese ripening, manufacture of cheeselike products, and lipolysis of butterfat and cream. Sources of lipases for cheese enhancement are the pancreatic glands or pregastric tissues of lamb, calf, or kid. Each pregastric lipase leads to its own characteristic flavor pattern, and these enzymes are essential in the production of quality cheeses such as Romano and provolone [15]. Pregastric lipases have also been used for the treatment of calf diarrhea or scours [15] and have potential for the treatment of malabsorption syndrome in children. 

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