Esters metabolism

As an example, the low-density lipoprotein (LDL) molecule and its receptor (Chapter 25) are internalized by means of coated pits containing the LDL receptor. These endocytotic vesicles containing LDL and its receptor fuse to lysosomes in the cell. The receptor is released and recycled back to the cell surface membrane, but the apoprotein of LDL is degraded and the choles-teryl esters metabolized. Synthesis of the LDL receptor is regulated by secondary or tertiary consequences of pinocytosis, eg, by metabolic products—such as choles-... 

Little has been done to study phthalate ester metabolism in other individual species. Mosquito larvae and salt marsh caterpillar larvae, however, were each shown to transform dioctyl... 

The elucidation of sinapoyl ester metabolism was aided by the availability of mutants. The sngl sinapoyl glucose accumulator 1) mutant of Arabidopsis had been identified based on a mutant screen for alterations in the composition of fluorescent compounds in the leaves. The screen was performed by thin layer chromatography and revealed that the leaves of the sngl mutant contained less sinapoylmalate and instead accumulated the precursor sinapoyl glucose (Lorenzen et al. 1996). 

Figure 3-13. Sinapoyl ester metabolism catalyzed by the enzymes (a) UDP-glucose sinapic acid glucosyltransferase (SGT), (b) sinapoylglucose malate sinapoyltransferase (SMT), (c) sinapoylglucosexholine sinapoyltransferase (SCT), and (d) sinapoylcholinesterase (SCE).

Lorenzen, M., Racicot, V., Stack, D., Chappie, C., 1996, Sinapic acid ester metabolism in wild type and a sinapoylglucose-accumulating mutant of Arabidopsis, Plant Physiol. 112 1625-1630. 

The important role of sterol components in the acquisition of freezing tolerance has recently been demonstrated using an Arabidopsis mutant with altered steryl-ester metabolism. Compared with the wild-type the mutant has no visible phenotype at standard growth temperature but exhibits clear symptoms when exposed to low temperatures (Hugly et al.,... 

Hugly, S., McCourt, P., Browse, J., Patterson, G.W. Somerville, C. (1990). A chilling sensitive mutant of Arabidopsis with altered steryl-ester metabolism. Plant Physiology 93, 1053-62. 

As shown in Figure 17.9, esters of sulfuric acid exist in which either one or both of the ionizable H atoms are replaced by hydrocarbon substituents, such as the methyl group. Replacement of one H yields an acid ester, and replacement of both yields an ester. Metabolically, acid ester sulfates are synthesized in phase II reactions to produce water-soluble products of xenobiotic compounds (such as phenol) that are readily eliminated from the body (see Section 7.4). 

It is, of course, also possible that the esterified cholesterol formed in HDL may be removed from plasma by some process other than uptake of the whole HDL particle or LTP-I-mediated transfer to other lipoprotein particles, but this possibility has not been fully investigated. Some evidence that there may be other pathways than these for the removal from plasma of HDL esterified cholesterol comes from the studies of Class et al. (G5, G6), who showed that cholesteryl ether incorporated in rat HDL as a tracer for cholesteryl ester was taken up in vivo by the rat liver (and by other organs) fester than apoA-I tracer (see Section 4.1.2). These studies are complicated by the relatively high concentration of apoE in rat HDL (compared, for instance, to man) and the unknown effect of apoE on HDL cholesteryl ester metabolism in the rat. Further studies on the removal of esterified cholester-... 

MiUcowski, G., Baumert, A., Schmidt, D., Nehlin, L. and Strack, D. (2004) Molecular regulation of sinapate ester metabolism in Brassica napus expression of genes, properties of the encoded proteins and correlation of enzyme activities with metabolite accumulation. Plant., 38, 80-92. 

All phthalate esters are readily absorbed, but toxicokinetics vary based on the route of exposure. Once absorbed, they are quickly distributed to organs and other body tissues such as the liver (bile) or kidneys. Phthalate esters metabolize quickly to a monoester but do not progress further. From 4.5% to 15% of single doses of 10-30 g of DEHP are excreted as metabolites in the urine of man. 

Melby J. C., St Cyr, M. Comparative studies on absorption and metabolic disposal of water-soluble corticosteroid esters. Metabolism 1961, 10, 75-82. 

Recently we have performed experiments in order to study the effects of a single dose (acute effects) and repeated administration of TTA (long-term effects) on the expression of rat hepatic genes related to peroxisomal and mitochondrial P-oxidation. In the acute study we have shovm that the expression of several mitochondrial and peroxisomal fatty acyl-CoA ester metabolizing enzymes were rapidly increased in rats fed TTA, but the increase seemed to follow different time-courses. Interestingly, CPT-11 mRNA and, especially, CPT-1 mRNA were induced early, that is at 4 and 2h, respectively, after TTA administration (Fig. 2). 

The purification and properties of LCAT, together with a discussion of its mechanism of reaction are given by Marcel (1982). A number of disease states involve LCAT activity. Familial LCAT deficiency has been described (Glomset and Norum, 1973) and patients with this rare complaint have been thoroughly investigated. Many of the abnormalities seem in such patients have been found in those with cholestasis also. A discussion of cholesterol ester metabolism in relation to other liver diseases and dyslipoproteinaemia has been reported (Marcel, 1982). Similarly, the metabolism of cholesterol esters in relation to arteries and arterial disease has been fully discussed (Kritchevsky and Kothari, 1978). Mammalian steroid sulphates have been reviewed by Farooqui (1981). 

Work on sterol ester metabolism in insects has been reviewed by Thompson et al. (1973). 

If the required plot of v against u/[s] is difficult to obtain because the experimental points exhibit excessive scatter, this may be an indication that the enzyme preparation is too impure for accurate kinetic measurements. The source of interference should be investigated. For example, liver esterases should be inhibited when measurements of ester metabolism are made, for example, by cytochrome P-450. The partially purified mixed enzyme character of a given subcellular fraction may prove to be an embarrassment at this time. 

SECTION 28 CARBOXYLIC ACID OR ESTER METABOLISM [NOTE 1 The term acid covers acidic ions and salts.] [NOTE 2 Ester hydrolysis questions follow the Table.] [NOTE 3 The items in the Table have the following forms Acetic acid is utilized. 

An allergic reaction to specific agents is an obvious contraindication. Allergy to para-aminobenzoic acid (PABA) is a contraindication to use of ester local anesthetics due to the fact that PABA is a metabolic product of ester metabolism. Methylparaben is a common preservative chemically similar to PABA and likewise can cause an allergic reaction. Metabisulfite is a commonly used preservative that may also cause allergic reactions but more notably is neurotoxic when used intrathecally. Local anesthetics containing any preservative should not be used intrathecally. Ester local... 

Much information is available about the metabolism of chylomicron cholesteryl esters taken up by the liver in association with the chylomicron remnant. This information may be relevant to the issue of chylomicron retinyl ester metabolism in the liver, about which much less direct information is on hand. Hepatic uptake of chylomicron cholesteryl esters occurs without hydrolysis of the cholesteryl esters (Goodman, 1965 (Juarfordt and Goodman, 1967 Stein et al., 1969). In studies with chylomicrons containing doubly labeled cholesteryl esters injected intravenously into rats, Quarfordt and Goodman (1967) observed that 80-90% of the chylomicron cholesteryl esters were removed by the liver without hydrolysis. In the liver, the newly absorbed cholesteryl esters underwent slow but extensive hydrolysis, to the extent of about 60% after 1 h and about 85-90% after 3.5 h. Subsequent to hydrolysis, most of the labeled free cholesterol slowly left the liver and was extensively redistributed in die body. Thus, 24 h later, only 20-28% of the labeled cholesterol found in the entire animal body was present in the liver. Since newly absorbed retinol, which is retained in the liver, is only mobilized slowly (see below), it is clear that following ester hydrolysis the hepatic metabolism of chylomicron cholesterol and retinol diverge in a major way. 

Hydrolysis of retinyl acetate by highly purified carboxylesterase from liver of several species (pig, ox, man) was reported by Bertram and Krisch (1969). The purified enzyme did not hydrolyze long-chain fatty acid esters of retinol. These findings suggested that previously observed hydrolysis of retinyl acetate by crude liver preparations was due to nonspecific esterases of little or no physiological relevance to hepatic retinyl ester metabolism. 

Milkowski and Strack (2010). The evolution of sinapate ester metabolism and strategies of targeted metabolic engineering, designed to generate low-sinapate ester lines of B. napus, have been discussed. 

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