Hydrolysis of ester linkages

Ester linkage is another important polar functionality in kerogen (Siskin et al. 1995) and the following hydrolysis reactions have been studied  

The results indicate that both ether and ester linkages, which are normally 

Because kinetic isotope effects depend on the hydrocarbon reaction mechanism, one would expect different isotopic fractionations involving water-organic interaction. Some preliminary results (Xiao 1998) on the carbon isotopic fractionations associated with hydrolysis reaction indicate that they are much smaller than those associated with HC thermal cracking. This suggests that a different isotopic model is needed for oil and gas generation in many environments with high water concentration. 

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The esterases are involved in the hydrolysis of ester linkages of various types. The products formed are acid and alcohol. These enzymes may hydrolyze triglycerides and include several lipases for instance, phospholipids are hydrolyzed by phospholipases, and cholesterol esters are hydrolyzed by cholesterol esterase. The carboxylesterases are enzymes that hydrolyze triglycerides such as tributyrin. They can be distinguished from lipases because they hydrolyze soluble substrates, whereas lipases only act at the water-lipid interfaces of emulsions. Therefore, any condition that results in increased surface area of the water-lipid interface will increase the activity of the enzyme. This is the reason that lipase activity is much greater in homogenized (not pasteurized) milk than in the non-homogenized product. Most of the lipolytic enzymes are specific for either the acid or the alcohol moiety of the substrate, and, in the case of esters of polyhydric alcohols, there may also be a positional specificity. 

If the classification of lipides were to be examined, some points would have to be made clear. In the first place, a definition of lipides should be made that excludes esters of mineral acids such as orthophosphoric acid. The phosphoric esters of alcohols and phenols are alkalino-stable—excluding those possessing a free carbonyl group moreover, they are not very soluble in organic solvents. The definition of lipides should therefore take into account the hydrolysis of ester linkage by heating with alkalies and the solubility of lipides in some organic solvents. 

Figure 1 Hydrolysis of ester linkages in glvcerophospholipids bv phospholipases Ai. A . C and D. The fatty acid at the sn-1-position (Ri) is usually saturated while that at the sn-2-position (R2) is unsaturated. R3 varies in different phospholipids and could be choline (phosphatidylcholine), ethanolamine (phosphatidylethanolamine), serine (phospha-tidylserlne), myoinositol (phosphatidylinositol), myoino-sitol-4-monophosphate (phosphatidylinositol-4-monophosphate) or myoinositol-4,5-blsphosphate (phosphatidylinositol-4,5-bisphosphate). In phosphatidic acid, R3 is a hydrogen atom.

Hydrolysis of ester linkages has been found to occur in all animals and bacteria however, the relative rates of hydrolysis in various species can be significantly different. In the case of local anesthetics, their metabohc disposition is of great practical importance since their toxicity depends largely on the balance between their rate of absorption and their rate of destruction. In most animals the esterases which hydrolyze many local anesthetics occur both in the liver and in the plasma. In the case ot human plasma, esterase activity is high and is principally responsible for the inactivation of procaine [60]. Cocaine is principally destroyed by liver esterase in the human, whereas plasma esterase in the rabbit is responsible for the hydrolysis of cocaine. The horse has little effective esterase activity against procaine in either plasma or liver and manifests central nervous system stimulation because of the slow destruction of the drug [61]. 

Tables 18.1,18.2, and 18.3 hst concentration measurements madeatdiEferenttimes during regular classes, final exams, and the summer when few students were present, respectively. A -Tetrahydrocannabinol (THC) is the major psychoactive compound in marijuana. THC is rapidly oxidized to 11-hydroxy THC and then to ll-nor-9-carboxy-A -THC, which is the main metabolite excreted. The main metabolic route of cocaine involves the hydrolysis of ester linkages to produce benzoylecgonine (BE). 6-Acetylmorphine is the main specific metabolite of heroin (3,6-diacetylmorphine). Codeine is O3 -methylmorphine. Morphine-3 p-glucuronide is a metabolite formed in vivo by the attachment of a sngar to the O3 oxygen, which facilitates elimination of morphine from the body. All three of these morphine-based derivatives are narcotics. 3,4-Methylenedioxymethamphetamine (MDMA) and 3,4-metltylenedioxy-amphetamine (MDA) are amphetamine derivatives that are commonly referred to as ecstasy.

Kinetic model for absorption and metabolism. In the intestinal tract, (V) is absorbed in an intact form but is partially metabolized by two processes (Scheme 7.1). One process is a reduction, being considered nonenzymatic, to form o-benzoylthiamine (VII), the other is an enzymatic hydrolysis of ester linkage (deacylation) to form the less absorbable (VI). From [118]. 

Proteolytic enzymes (proteases) catalyse the hydrolysis of peptide (amide) bonds and sometimes the related hydrolysis of ester linkages. Proteases are divided into four groups on the basis of their mechanism of action ... 

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