Endoplasmic reticulum, cholesterol synthesis

A little more than half the cholesterol of the body arises by synthesis (about 700 mg/d), and the remainder is provided by the average diet. The liver and intestine account for approximately 10% each of total synthesis in humans. Virtually all tissues containing nucleated cells are capable of cholesterol synthesis, which occurs in the endoplasmic reticulum and the cytosol. 

The lipid compositions of plasma membranes, endoplasmic reticulum and Golgi membranes are distinct 26 Cholesterol transport and regulation in the central nervous system is distinct from that of peripheral tissues 26 In adult brain most cholesterol synthesis occurs in astrocytes 26 The astrocytic cholesterol supply to neurons is important for neuronal development and remodeling 27 The structure and roles of membrane microdomains (rafts) in cell membranes are under intensive study but many aspects are still unresolved 28... 

Formation of mevalonate. The conversion of acetyl CoA to acetoacetyl CoA and then to 3-hydroxy-3-methylglutaryl CoA (3-HMG CoA) corresponds to the biosynthetic pathway for ketone bodies (details on p. 312). In this case, however, the synthesis occurs not in the mitochondria as in ketone body synthesis, but in the smooth endoplasmic reticulum. In the next step, the 3-HMG group is cleaved from the CoA and at the same time reduced to mevalonate with the help of NADPH+H 3-HMG CoA reductase is the key enzyme in cholesterol biosynthesis. It is regulated by repression of transcription (effectors oxysterols such as cholesterol) and by interconversion... 

The endoplasmic reticulum is composed of a convoluted network of channels and so has a large surface area. Apart from cytochromes P-450, the endoplasmic reticulum has many enzymes and functions, besides the metabolism of foreign compounds. These include the synthesis of proteins and triglycerides and other aspects of lipid metabolism and fatty acid metabolism. Specific enzymes present on the endoplasmic reticulum include cholesterol esterase, azo reductase, glucuronosyl transferase, NADPH cytochromes P-450 reductase and NADH cytochrome b5 reductase and cytochrome b5. A FAD-containing monooxygenase is also found in the endoplasmic reticulum, and this is discussed later in this chapter. 

The synthesis of the apoproteins takes place on ribosomes that are bound to the endoplasmic reticulum. As we mentioned previously, the biosynthesis of the other lipids in lipoproteins (cholesterol, triacylglycerols, and phospholipids) also occurs on the endoplasmic reticulum. 

Biosynthesis of cholesteryl esters. The acyl-CoA cholesterol acyltransferase involved in cholesteryl ester synthesis is located on the cytosolic surface of liver endoplasmic reticulum. 

Postulated scheme for the synthesis, assembly, and secretion of VLDL by a hepatocyte (liver cell). (1) Synthesis The apoproteins, phospholipid, triacylglycerol, cholesterol, and cholesteryl esters are synthesized in the endoplasmic reticulum. (2) Assembly These components are assembled into a prelipoprotein particle in the lumen of the endoplasmic reticulum. (3) Processing The particle moves to the Golgi apparatus, where modification of the apoproteins occurs. 

Slightly less than half of the cholesterol in the body derives from biosynthesis de novo biosynthesis in the liver accounts for approximately 10% and in the intestines approximately 15% each day. Cholesterol synthesis occurs in the cytoplasm and microsomes (smooth endoplasmic reticulum) (Figure 6.4). The process has five major steps ... 

CYP enzymes are a large and diverse superfamily of haemoproteins. Primarily membrane-associated proteins, they are located in the inner membrane of mitochondria and the endoplasmic reticulum of cells, and metabolise thousands of endogenous and exogenous compounds. Most of these enzymes can metabolise multiple substrates, and many can catalyse multiple reactions. While prevalent in the liver, CYP enzymes are also present in most other tissues of the body, and play an important role in hormone synthesis and breakdown (including oestrogen and testosterone synthesis and metabolism), cholesterol synthesis and vitamin D metabolism. Hepatic CYPs are the most widely smdied. 

Abstract. When experimental animals are kept on an atherogenic diet the NADPH-dependent phospholipid deoxygenase in the membranes of the hepatic endoplasmic reticulum is activated and the degree of membrane oxidation is increased. Peroxide modification of microsomal membranes is attended by changes in their conformation and as a consequence, changes in the activity of membrane-bound enzymes. Proceeding from the fact that the synthesis of the components and the assembly of the supramolecular lipoprotein structure as well as cholesterol catabolism are accomplished by the enzyme systems localized in the hepatic microsomes, the role of peroxidation of the microsomal lipids in the pathogenesis of atherosclerosis is discussed. 

Fig. 3. Summary of key mechanisms of action through which a model adrenotoxicant (indicated by a black star) could disrupt the synthesis of corticosteroids. References presenting data in support of this model are given in the text. ACTH, adrenocorticotropic hormone Rc, receptor G, G-protein AC, adenylyl cyclase Ca, calcium ATP, adenosine triphosphate cAMP, cyclic adenosine monophosphate PKA, protein kinase A StAR, Steroid acute regulatory protein SCC, P450SCO, cholesterol side chain cleaving enzyme 11/3, 11/3-hydroxylase 17a, 17a-hydroxylase 3/3-HSD, 3/3-hydroxysteroid-5A-steroid dehydrogenase C21, 21-hydroxylase ER, endoplasmic reticulum.

HMG-CoA reductase is an integral protein of the endoplasmic reticulum and the primary site of regulation of synthesis of cholesterol and nonsterol isoprenoid derivatives. Its activity has a well-defined diurnal rhythm in rats and mice, coinciding with that of the enzyme s synthesis and of the mRNA concentration. Activity is highest at about the middle of the dark period and lowest at about the middle of the light period. Its mechanism may be related to food consumption. Rats are nocturnal animals and consume food in the dark the increased bile production and excretion depletes liver cholesterol and may stimulate the increased synthesis of HMG-CoA reductase as a compensatory mechanism. 

The enzymes responsible for the conversion of lanosterol to cholesterol, as were those for the conversion of farnesyl pyrophosphate to squalene and lanosterol, are all integral membrane-bound proteins of the endoplasmic reticulum. Many have resisted solubiUzation, some have been partially purified, and several have been obtained as pure proteins. As a consequence, much of the enzymological and mechanistic studies have been done on impure systems and one would anticipate a more detailed and improved understanding of these events as more highly purified enzymes become available. Many approaches have been taken to establish the biosynthetic route that sterols follow to cholesterol. Some examples are synthesis of potential intermediates, the use of inhibitors, both of sterol transformations and of the electron transfer systems, and by isotope dilution experiments. There is good evidence that the enzymes involved in these transformations do not have strict substrate specificity. As a result, many compounds that have been found to be converted to intermediates or to cholesterol may not be true intermediates. In addition, there is structural similarity between many of the intermediates so that alternate pathways and metabolites are possible. For example, it has been shown that side-chain saturation can be either the first or the last reaction in the sequence. Fig. 21 shows a most probable series of intermediates for this biosynthetic pathway. 

The changes in activity and/or properties of HMG-CoA reductase reported above are, however, accompanied by large changes in enzyme quantity typical of such feeding regimens. These would appear to overshadow any effects due to membrane compositional changes. However, the hypothesis that cholesterol directly feedback-inhibits its own synthesis by altering the catalytic activity of HMG-CoA reductase remains attractive since newly synthesized cholesterol appears preferentially in the smooth endoplasmic reticulum that harbors HMG-CoA reductase [115],... 

A FIGURE 18-1 Overview of synthesis of major membrane lipids and their movement into and out of cells. Membrane lipids (e.g., phospholipids, cholesterol) are synthesized through complex multienzyme pathways that begin with sets of water-soluble enzymes and intermediates in the cytosol (D) that are then converted by membrane-associated enzymes into water-insoluble products embedded in the membrane (B), usually at the interface between the cytosolic leaflet of the endoplasmic reticulum (ER) and the cytosol. Membrane lipids can move from the ER to other organelles (H), such as the Golgi apparatus or the mitochondrion, by either vesicle-mediated or other poorly defined mechanisms. Lipids can move into or out of cells by plasma-membrane transport proteins or by lipoproteins. Transport proteins similar to those described in Chapter 7 that move lipids (0) include sodium-coupled symporters that mediate import CD36 and SR-BI superfamily proteins that can mediate... 

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