Reticulum, endoplasmic

The total amount of sialic acids in the smooth and rough endoplasmic reticulum (ER) are difficult to estimate accurately because of the inadequacy of current methods of isolation of these fractions, both in terms of purity and yield. Microsomal fractions are highly complex in their composition and consist of ER, surface membrane, and probably Golgi membranes and other organelles. It is difficult to estimate how much of the sialic acid actually belongs to endoplasmic reticulum. It is present, but the concentration is certainly less than in plasma membrane. Further, it can be seen in Table V (Ehrlich ascites tumor and 

Homogenate Plasma membrane Smooth internal membranes Ribosome bearing membranes 

Whole cells Plasma membrane Lysosomes Mitochondria 

Chick embryo Fibroblast (CEF) Homogenate Surface membrane 

CEF-RBA (avian sarcoma virus) Homogenate Surface membrane 

It has become increasingly clear that there is a non-mitochondrial, intracellular calcium pool which plays an important role in cell activation in a large number of nonmuscle cells as well as in smooth and skeletal muscle. This pool is relatively enormous in skeletal muscle, provides the bulk of the Ca2+ needed to regulate skeletal muscle contraction, and is located in a distinct organelle, the sarcoplasmic reticulum. The pool is smaller in non-muscle and in smooth muscle cells, and its location less obvious [10,11]. To fill the pool requires ATP, i.e., uptake of Ca2+ into the pool is driven by a distinct Ca2+-ATPase, an enzyme which purifies with the mi- 

The mitochondrial Ca2+ pool plays a second role in cellular Ca2+ homeostasis by serving as a sink for Ca2+ during times of excessive Ca2+ uptake by the cell. Under this circumstance, the non-ionic calcium pool in the matrix space can increase 10-fold or more, thereby protecting the cell from Ca2+ intoxication. This mechanism provides a temporary device by which the cell can protect itself, but in the long term only by regulating Ca2+ fluxes across the plasma membrane can the cell maintain Ca2+ homeostasis [14]. 

The cytoplasm usually has a thin peripheral area in contact with cell membrane, termed the ectoplasm. This area has few organelles and inclusion bodies, which tend to be located in the central part of the cytoplasm, named the endoplasm. 

Although the basic architecture of all eukaryotic cells is formed by membranes, organelles, and cytosol, each cell type exhibits a distinct morphology defined by cell shape and localization of organelles. The structural basis of the characteristic morphology of each cell type is the cytoskeleton, a dense network of three classes of filamentous proteins that permeate the cytosol and support the cell membrane. 

This is an interconnected network of flattened or spherical vesicles and tubules found in the cytoplasm of eukaryotic cells. These structures are enveloped by a membrane that separates the endoplasmic reticulum cavities or cisternae. The cisternae constitute a network of channels that go through the cytoplasm and regulate the transport of various cell products, generally to the exterior environment. In some cells the cisternae also serve as a storage area. There are two types of endoplasmic reticulum granular (or rough) and smooth. 

The rough endoplasmic reticulum contains ribosomes on the surfaces, a cell structure that will be discussed in the next section. The system formed by endoplasmic reticulum and ribosomes is associated with protein synthesis. The endoplasmic reticulum also participates in lipid biosynthesis. In different types of eukaryotic cells, the endoplasmic reticulum has different forms and functions. In muscle cells, in which Ca2+ stimulates contraction, the endoplasmic reticulum participates in the relaxation process, reabsorbing Ca2+ ions. 

The smooth endoplasmic reticulum also consists of a network of tubules that is considerably developed in certain types of special cells, such as those that secrete steroid hormones. 

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CH2SH + 1/2 O2 -CH2-S-S-CH2 + H2O This reaction requires an oxidative environment, and such disulfide bridges are usually not found in intracellular proteins, which spend their lifetime in an essentially reductive environment. Disulfide bridges do, however, occur quite frequently among extracellular proteins that are secreted from cells, and in eucaryotes, formation of these bridges occurs within the lumen of the endoplasmic reticulum, the first compartment of the secretory pathway. 

RBP is synthesized in the hepatocytes, where it picks up one molecule of retinol in the endoplasmic reticulum. Both its synthesis and its secretion from the hepatocytes to the plasma are regulated by retinol. In plasma, the... 

Collagen chains are synthesized as longer precursors, called procollagens, with globular extensions—propeptides of about 200 residues—at both ends. These procollagen polypeptide chains are transported into the lumen of the rough endoplasmic reticulum where they undergo hydroxylation and other chemical modifications before they are assembled into triple chain molecules. The terminal propeptides are essential for proper formation of triple... 

Brefeldin A, an antiviral agent which impedes protein transport from the endoplasmic reticulum to the Golgi complex, was synthesized as the racemate using a number of interesting diastereoselective reactions. 

Nucleus, Mitochondria, Chloroplasts, Endoplasmic reticulum, Golgi apparatus. Vacuole... 

Endoplasmic reticulum Flattened sacs, tubes, and sheets of internal The endoplasmic reticulum is a labyrinthine... 

Golgi apparatus, endoplasmic reticulum, ribosomes, lysosomes, peroxisomes, and cytoskeleton... 

Ribosomes and microsomes consisting of endoplasmic reticulum, Golgi, and plasma membrane fragments... 

FIGURE 23.8 Glu cose-6-phosphatase is localized in the endoplasmic reticulum membrane. Conversion of glucose-6-phosphate to glucose occurs during transport into the ER. 

In the endoplasmic reticulum of eukaryotic cells, the oxidation of the terminal carbon of a normal fatty acid—a process termed ch-oxidation—can lead to the synthesis of small amounts of dicarboxylic acids (Figure 24.27). Cytochrome P-450, a monooxygenase enzyme that requires NADPH as a coenzyme and uses O, as a substrate, places a hydroxyl group at the terminal carbon. Subsequent oxidation to a carboxyl group produces a dicarboxylic acid. Either end can form an ester linkage to CoA and be subjected to /3-oxidation, producing a... 

As seen already, palmitate is the primary product of the fatty acid synthase. Cells synthesize many other fatty acids. Shorter chains are easily made if the chain is released before reaching 16 carbons in length. Longer chains are made through special elongation reactions, which occur both in the mitochondria and at the surface of the endoplasmic reticulum. The ER reactions are actually quite similar to those we have just discussed addition of two-carbon units... 

This impressive reaction is catalyzed by stearoyl-CoA desaturase, a 53-kD enzyme containing a nonheme iron center. NADH and oxygen (Og) are required, as are two other proteins cytochrome 65 reductase (a 43-kD flavo-protein) and cytochrome 65 (16.7 kD). All three proteins are associated with the endoplasmic reticulum membrane. Cytochrome reductase transfers a pair of electrons from NADH through FAD to cytochrome (Figure 25.14). Oxidation of reduced cytochrome be, is coupled to reduction of nonheme Fe to Fe in the desaturase. The Fe accepts a pair of electrons (one at a time in a cycle) from cytochrome b and creates a cis double bond at the 9,10-posi-tion of the stearoyl-CoA substrate. Og is the terminal electron acceptor in this fatty acyl desaturation cycle. Note that two water molecules are made, which means that four electrons are transferred overall. Two of these come through the reaction sequence from NADH, and two come from the fatty acyl substrate that is being dehydrogenated. 

Organisms differ with respect to formation, processing, and utilization of polyunsaturated fatty acids. E. coli, for example, does not have any polyunsaturated fatty acids. Eukaryotes do synthesize a variety of polyunsaturated fatty acids, certain organisms more than others. For example, plants manufacture double bonds between the A and the methyl end of the chain, but mammals cannot. Plants readily desaturate oleic acid at the 12-position (to give linoleic acid) or at both the 12- and 15-positions (producing linolenic acid). Mammals require polyunsaturated fatty acids, but must acquire them in their diet. As such, they are referred to as essential fatty acids. On the other hand, mammals can introduce double bonds between the double bond at the 8- or 9-posi-tion and the carboxyl group. Enzyme complexes in the endoplasmic reticulum desaturate the 5-position, provided a double bond exists at the 8-position, and form a double bond at the 6-position if one already exists at the 9-position. Thus, oleate can be unsaturated at the 6,7-position to give an 18 2 d5-A ,A fatty acid. 

Mammals synthesize phosphatidylserine (PS) in a calcium ion-dependent reaction involving aminoalcohol exchange (Figure 25.21). The enzyme catalyzing this reaction is associated with the endoplasmic reticulum and will accept phosphatidylethanolamine (PE) and other phospholipid substrates. A mitochondrial PS decarboxylase can subsequently convert PS to PE. No other pathway converting serine to ethanolamine has been found. 

All prostaglandins are cyclopentanoic acids derived from arachidonic acid. The biosynthesis of prostaglandins is initiated by an enzyme associated with the endoplasmic reticulum, called prostaglandin endoperoxide synthase, also known as cyclooxygenase. The enzyme catalyzes simultaneous oxidation and cyclization of arachidonic acid. The enzyme is viewed as having two distinct activities, cyclooxygenase and peroxidase, as shown in Figure 25.28. 

Squalene monooxygenase, an enzyme bound to the endoplasmic reticulum, converts squalene to squalene-2,3-epoxide (Figure 25.35). This reaction employs FAD and NADPH as coenzymes and requires Og as well as a cytosolic protein called soluble protein activator. A second ER membrane enzyme, 2,3-oxidosqualene lanosterol cyclase, catalyzes the second reaction, which involves a succession of 1,2 shifts of hydride ions and methyl groups. 

Although lanosterol may appear similar to cholesterol in structure, another 20 steps are required to convert lanosterol to cholesterol (Figure 25.35). The enzymes responsible for this are all associated with the endoplasmic reticulum. The primary pathway involves 7-dehydroeholesterol as the penultimate intermediate. An alternative pathway, also composed of many steps, produces the intermediate desmosterol. Reduction of the double bond at C-24 yields cholesterol. Cholesterol esters—a principal form of circulating cholesterol—are synthesized by acyl-CoA cholesterol acyltransferases (ACAT) on the cytoplasmic face of the endoplasmic reticulum. 

HDL and VLDL are assembled primarily in the endoplasmic reticulum of the liver (with smaller amounts produced in the intestine), whereas chylomicrons form in the intestine. LDL is not synthesized directly, but is made from VLDL. LDL appears to be the major circulatory complex for cholesterol and cholesterol esters. The primary task of chylomicrons is to transport triacylglycerols. Despite all this, it is extremely important to note that each of these lipoprotein classes contains some of each type of lipid. The relative amounts of HDL and LDL are important in the disposition of cholesterol in the body and in the development of arterial plaques (Figure 25.36). The structures of the various... 

Kendall, J. M., et al. (1996). Recombinant apoaequorin acting as a pseudo-luciferase reports micromolar changes in the endoplasmic reticulum free Ca2+ of intact cells. Biochem. J. 318 383-387. 

Proteins embedded in the shell of lipoproteins. They serve as scaffold for assembly of the lipoprotein particle in the endoplasmic reticulum. In addition, they control metabolism of lipoproteins in the circulation by interaction with enzymes such as lipases. Finally, apolipoproteins determine cellular uptake of the particles by interaction with specific lipoprotein receptors expressed on the surface of target cells. 

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