Endoplasmic reticulum General

In the sarcoplasm of smooth muscle cells there is a membrane bound compartment usually referred to as the SR by analogy with skeletal muscle. However, it is not at all clear that the interior of these membrane-bound regions are continuous as they are in skeletal muscle. The primary properties of this system seem to be quite similar to those of the endoplasmic reticulum of many other cell types. In general, calcium is concentrated into the membrane-bound reticulum and then released to initiate the characteristic action of the cell. 

Their hydroxylated products are more water-soluble than their generally lipophilic substrates, facilitating excretion Liver contains highest amounts, but found In most If not all tissues. Including small Intestine, brain, and lung Located in the smooth endoplasmic reticulum or in mitochondria (steroidogenic hormones)... 

Cytochrome P450s are generally located in the endoplasmic reticulum of cells and are particularly enriched in liver. 

The coupling of solute transport in the GI lumen with solute lumenal metabolism (homogeneous reaction) and membrane metabolism (heterogeneous reaction) has been discussed by Sinko et al. [54] and is more generally treated in Cussler s text [55], At the cellular level, solute metabolism can occur at the mucosal membrane, in the enterocyte cytosol, and in the endoplasmic reticulum (or microsomal compartment). For peptide drugs, the extent of hydrolysis by lumenal and membrane-bound peptidases reduces drug availability for intestinal absorption [56], Preferential hydrolysis (metabolic specificity) has been targeted for reconversion... 

The source of free radicals is multiplied under these circumstances, arachidonic acid metabolism, activation of xanthine oxidase, perturbation of electron flow within the respiratory chain, and NOS activation. Structurally, excitotoxicity is generally described as a necrotic process involving initial swelling of the cell and of the endoplasmic reticulum, clumping of chromatin, followed by swelling of the... 

Proteins produced in plant cells can remain within the cell or are secreted into the apoplast via the bulk transport (secretory) pathway. In whole plants, because levels of protein accumulated intracellularly, e. g. using the KDEL sequence to ensure retention in the endoplasmic reticulum, are often higher than when the product is secreted [58], foreign proteins are generally not directed for secretion. However, as protein purification from plant biomass is potentially much more difficult and expensive than protein recovery from culture medium, protein secretion is considered an advantage in tissue culture systems. For economic harvesting from the medium, the protein should be stable once secreted and should accumulate to high levels in the extracellular environment. 

Both the CYP and the reductase are firmly embedded within the membrane of the smooth endoplasmic reticulum (SER), which forms microsomes when cells are broken for experimental purposes. Figure 6.28 illustrates the whole process of a generalized phase I hydroxylation. 

Figure 2. General topological feature of PS translocation and decarboxylation in mammalian cells. PS is synthesized by PSS I and II in endoplasmic reticulum (ER) or mitochondria-associated membrane (MAM). The nascent PS is transported other membranes such as plasma membrane, nucleus, and mitochondria. The PS transported to the mitochondrial outer membrane is then translocated to the inner membrane, in which PS is converted to PE by PS decarboxylase (PSD). The PE formed in mitochondria is dynamic and can be exported to other organelles for membrane biogenesis.

Cytotoxicity is the general term used to describe toxicity at the level of the cell. It can be brought about in many ways, usually by a chemical interaction between the toxic agent and one or more components of the cell. Interactions can be permanently damaging or may lead to temporary injury that the cell is capable of repairing. Perhaps the most important sites of intracellular injury are cell membranes, the cell nucleus (home of DNA), mitochondria (home of energy production), and endoplasmic reticulum (home of the biosynthesis of the all-important protein molecules, essential for cell structure and, as enzymes, for the catalysis of all cellular reactions and for the metabolism of foreign chemicals). 

Death by apoptosis is much less spectacular and is usually quicker than necrosis. Cells shrink, possibly to a quarter of their previous size, and vesicles appear in the cytosol and in the nucleus. As the cytosol contracts, the chromatin fragments into a number of distinct particles and the endoplasmic reticulum fuses with the plasma membrane. The dying cell breaks up into small vesicles (apop-totic bodies) which are removed by phagocytosis (Figure 20.34). Since little if any of the cytosolic contents escapes, there is little or no inflammation. That is, death by apotosis rather than reduces the risk of local or general inflammation. In fact, death by apoptosis is so inconspicuous that it was not discovered for many years. 

Many proteins on the surface of the plasma membrane, and the majority of secreted proteins, contain oligosaccharide residues that are post-translationally added to the endoplasmic reticulum and in the Golgi apparatus (see p.230). By contrast, cytoplasmic proteins are rarely glycosylated. Glycoproteins can contain more than 50% carbohydrate however, the proportion of protein is generally much greater. 

---