Cholesterol esters formation

Cholesterol absorption inhibition. Oleo-resin, administered orally to rats, was active on cholesterol-primed animals k Cholesterol ester formation. Decoction of the dried rhizome, administered intragastri-cally to mice at a dose of 1.2 g/kg, produced no effect. The study was conducted with a mixture of Zingiber officinale (Rh), Bupleu-rum falcatum (Rt), Scutellaria baicalensis (Rt), Pinellia ternata (Tu), Zizyphus jujuba (Fr), Panax ginseng (Rt), and Glycyrrhiza glabra (Rh) . 

Carrol, R.M. and Rudel, L.L. 1981. Dietary fat and cholesterol effects on lipoprotein cholesterol ester formation via lecithin-cholestrol acyltrans-ferase (LCAT) in vervet monkey. J. Lipid Res. 22 359-363. 

Finally, the enzymatic nature of CPIA-cholesterol ester formation will be briefly mentioned. None of the enzyme preparations of three known biosynthetic pathways for cholesterol esters, namely, acyl-CoA cholesterol Q-acyltransferase (ACAT), lecithin cholesterol 0-acyltransferase (LCAT), nor cholesterol esterase, was effective in producing CPIA-cholesterol ester from the Ba isomer or CPIA. In contrast, the 9,000 g supernatant or microsomal fractions from liver or kidney homogenate were found to be capable of producing CPIA-cholesterol ester without the addition of any cofactors. As substrate, only the Ba isomer was effective, and none of the 3 other fenvalerate isomers nor free CPIA was effective. The hepatic enzyme preparation also catalyzed hydrolysis of fenvalerate, and in this case all the 4 isomers were utilized as substrates. These facts imply that CPIA-cholesterol ester is formed from the Ba isomer through a transesterification reaction via intermediary acyl-enzyme complex. 

The enzyme activity responsible for CPIA-cholesterol ester formation is distributed in several tissues of animal species including rats, mice, dogs and monkeys, as shown in Table VI. No tissue enzymes were found to react with the Aa, a6 or b6 isomer. 

Table VI. CPIA-Cholesterol Ester Formation from the Btx Isomer...

Excess accumulation of free cholesterol in cells stimulates the rate of cholesterol ester formation and induces deposition of cholestryl ester inclusions in the cytoplasm similar to the situation in the foam cells of atherosclerotic plaque. 

As shown in Fig. 3, they studied the formation of cholesterol ester With time in rat liver microsomes in imidazole buffer. As can be seen, in the presence of magnesium there was a considerable decline in the amount of cholesterol ester formed, but that it could be completely prevented or blocked if potassium fluoride and EDTA were included in the incubation mixture. In a sepairate experiment, they studied cholesterol ester formation as a function of magnesium ion concentration in the medium. Again, the ACAT activity declined with increasing magnesium in the medium, whereas inclusion of EDTA together with magnesium, stabilized the enzyme activity. This information is consistent with the data from our laboratory which demonstrates the presence in microsomes of a Mg++ sensitive phosphatase which inactivates 7a-hydroxylase. 

Figure 12.7 Schematic diagram showing the role of LDL glycatlon and oxidation in foam-cell formation [adapted from Lyons (1991) and Esterbauer et al. (1992)]. C, cholesterol CE, cholesterol ester.

The rare example of synergistic action of a binary mixture of 1-naphthyl-A-phcnylaminc and phenol (1-naphthol, 2-(l,l-dimethylethyl)hydroquinone) on the initiated oxidation of cholesterol esters was evidenced by Vardanyan [34]. The mixture of two antioxidants was proved to terminate more chains than both inhibitors can do separately ( > /[xj). For example, 1-naphtol in a concentration of 5 x 10 5 mol L-1 creates the induction period t=170s, 1 -naphthyl-A-phenylamine in a concentration of 1.0 x 10-4 mol L 1 creates the induction period t = 400s, and together both antioxidants create the induction period r = 770 s (oxidation of ester of pelargonic acid cholesterol at 7= 348 K with AIBN as initiator). Hence, the ratio fs/ZfjXi was found equal to 2.78. The formation of an efficient intermediate inhibitor as a result of interaction of intermediate free radicals formed from phenol and amine was postulated. This inhibitor was proved to be produced by the interaction of oxidation products of phenol and amine. 

Belkner et al. [32] demonstrated that 15-LOX oxidized preferably LDL cholesterol esters. Even in the presence of free linoleic acid, cholesteryl linoleate continued to be a major LOX substrate. It was also found that the depletion of LDL from a-tocopherol has not prevented the LDL oxidation. This is of a special interest in connection with the role of a-tocopherol in LDL oxidation. As the majority of cholesteryl esters is normally buried in the core of a lipoprotein particle and cannot be directly oxidized by LOX, it has been suggested that LDL oxidation might be initiated by a-tocopheryl radical formed during the oxidation of a-tocopherol [33,34]. Correspondingly, it was concluded that the oxidation of LDL by soybean and recombinant human 15-LOXs may occur by two pathways (a) LDL-free fatty acids are oxidized enzymatically with the formation of a-tocopheryl radical, and (b) the a-tocopheryl-mediated oxidation of cholesteryl esters occurs via a nonenzymatic way. Pro and con proofs related to the prooxidant role of a-tocopherol were considered in Chapter 25 in connection with the study of nonenzymatic lipid oxidation and in Chapter 29 dedicated to antioxidants. It should be stressed that comparison of the possible effects of a-tocopherol and nitric oxide on LDL oxidation does not support importance of a-tocopherol prooxidant activity. It should be mentioned that the above data describing the activity of cholesteryl esters in LDL oxidation are in contradiction with some earlier results. Thus in 1988, Sparrow et al. [35] suggested that the 15-LOX-catalyzed oxidation of LDL is accelerated in the presence of phospholipase A2, i.e., the hydrolysis of cholesterol esters is an important step in LDL oxidation. 

Kaneko, H., M. Matsuo, and J. Miyamoto. 1986. Differential metabolism of fenvalerate and granuloma formation. I. Identification of a cholesterol ester derived from a specific chiral isomer of fenvalerate. Toxicol. Appl. Pharmacol. 83 148-156. 

Miyamoto, J., H. Kaneko, and Y. Takamatsu. 1986. Stereoselective formation of a cholesterol ester conjugate from fenvalerate by mouse microsomal carboxyesterase(s). Jour. Biochem. Toxicol. 1 79-94. 

In addition, three types of lipophilic conjugates have been found in pyrethroid metabolism studies (Fig. 4). They are cholesterol ester (fenvalerate) [15], glyceride (3-PBacid, a common metabolite of several pyrethroids) [16], and bile acid conjugates (fluvalinate) [17]. It is noteworthy that one isomer out of the four chiral isomers of fenvalerate yields a cholesterol ester conjugate from its acid moiety [15]. This chiral-specific formation of the cholesterol ester has been demonstrated to be mediated by transesterification reactions of carboxylesterase(s) in microsomes, not by any of the three known biosynthetic pathways of endogenous cholesterol esters... 

An old hypothesis is based on the observations of Dahlen et al. (D3), who demonstrated that above a certain concentration in plasma, Lp(a) could bind to glycosaminoglycans in the arterial wall (B12). Colocalization of Lp(a) and fibrin on the arterial wall can lead to oxidative changes in the lipid moiety of Lp(a) and induce the formation of oxidatively modified cholesterol esters, which in turn can influence the interaction of Lp(a) and its receptors on macrophages. This process is promoted by the presence of calcium ions. Cushing (C14), Loscalzo (L22), and Rath (R3) reported a colocalization of undegraded Lp(a) and apo-Bl00 in the extracellular space of the arterial wall. In contrast to LDL, Lp(a) is a substrate for tissue transglutaminase and Factor XUIa and can be altered to products that readily interact with cell surface structures (B21). 

There are a few reported cases of esterases that catalyze not only hydrolysis but also the reverse reaction of ester formation, in analogy with the global reaction described for serine peptidases (Fig. 3.4). Thus, cholesterol esterase can catalyze the esterification of oleic acid with cholesterol and, more importantly in our context, that of fatty acids with haloethanols [54], Esterification and transesterification reactions are also mediated by carboxyleste-rases, as discussed in greater detail in Sect. 7.4. 

Beauveriolides I (19) and III (20), two fungal (Beauveria Spp) metabolites, have been found to be specific inhibitors of lipid droplet formation in mouse macrophages. It has been recently observed that the metabolisms of A[3 proteins and cholesterol esters are closely linked. One of the... 

This effect may be due to decreasing hepatic cholesterol and cholesterol ester levels to such an extent that hepatic formation of VLDL is impaired. The statins also have been claimed to reduce blood cholesterol levels modestly in some patients with homozygous familial hypercholesterolemia, a condition often fatal in childhood or in early adulthood. 

ACAT transfers amino-acyl groups from one molecule to another. ACAT is an important enzyme in bile acid synthesis, and catalyses the intracellular esterification of cholesterol and formation of cholesteryl esters. ACAT-mediated esterification of cholesterol limits its solubility in the cell membrane and thus promotes accumulation of cholesterol ester in the fat droplets within the cytoplasm this process is important in preventing the toxic accumulation of free cholesterol that would otherwise damage ceU-membrane structure and function. Most of the cholesterol absorbed during intestinal transport undergoes ACAT-mediated esterification before incorporation into chylomicrons. In the liver, ACAT-mediated esterification of cholesterol is involved in the production and release of apo-B-containing lipoproteins. 

The structure of the blue phase is of some importance. Among the lipoproteins carrying lipids in the blood, low-density lipoproteins (LDL) have attracted much attention. They are the factors mainly responsible for plaque formation, which ultimately leads to atheriosclerotic changes and heart disease. The major components of the LDL-particles are cholesterol fatty acid esters. A remarlmble property is the constant size of LDL particles [28], which indicates that the interior must possess some degree of order. It seems probable that the structure proposed above for cholesterol esters in the cholesteric liquid-crystalline structure should occur also in the LDL-particle. In that case the LDL particle can be viewed as a dispersed blue phase, whose size is related to the periodicity of the liquid-crystalline phase, and the protein coat at the surface is oriented parallel to adjacent specific crystallographic planes of the blue phase. These amphiphilic proteins will expose lipophilic segments inwards emd expose hydrophilic groups towards tiie enviroiunent. 

---