Organelle membranes

Following the action of extraordinary stimulants (hypoxic hypoxia, hypoxia + hyperoxia, hypodynamia + hyperthermia), animals demonstrate an accumulation of malonic dialdehyde with a simultaneous fall of antiradical activity of the liver tissue. A preliminary introduction to rats of acetylene amine 3,4,5-tris(morpho-linopropynyl)-l-methylpyrazole 103 and also of tocopherol antioxidant and gutumine antihypoxant averts activation of the lipid peroxidation processes. The inhibition of peroxidation with this agent is mediated by stabilization of ly-zosomal and mitochondrial membranes. Unsaturated amines prevent destruction of the organelle membranes provoked by UV irradiation and incubation at 37°C (pH4.7)(78MIl). 

Hypothermia slows down enzyme catalysis of enzymes in plasma membranes or organelle membranes, as well as enzymes floating around in the cytosol. The primary reason enzyme activity is decreased is related to the decrease in molecular motion by lowering the temperature as expressed in the Arrhenius relationship (k = where k is the rate constant of the reaction, Ea the activation energy,... 

Desiccation tolerant species may exhibit little or no metabolic activity depending upon the extent of dehydration. In this anhydrobiotic or ametabolic state we are concerned not with metabolic perturbation but with the stability of organelles, membranes and macromolecules in a dehydrated state. However, in the initial period of rehydration, the passage to a metabolically active state poses particular problems if metabolic mayhem is to be avoided. 

Most transport vesicles bud off as coated vesicles, with a unique set of proteins decorating their cytosolic surface. The coat has two major known functions. First, it concentrates and selects specific membrane proteins in a discrete portion of donor organelle membrane that will serve as origin to the transport vesicle. Second, the assembly of coat proteins into curved structures delineates the area of the forming transport vesicle. The size and curvature is a function of the coat composition. Thus, vesicles with similar vesicle coat have closely similar size and shape [3]. 

Liver necrosis is another concern following hexachloroethane exposure. Hexachloroethane is metabolized in the centrilobular area of the liver by way of the microsomal mixed function oxidase system. The relatively nonpolar pentachloroethyl free radical is an intermediate in this pathway. The reaction of the free radical with unsaturated lipids in the cellular or organelle membranes could contribute to hepatocyte damage and necrosis. 

Most organelle membranes, such as the tonoplast (6) and the Golgi apparatus (7), can be separated by density gradient ultracentrifugation of plant cell homogenates. However, other effective methods for the isolation of the plasma membrane (8,9) have been described. Moreover, another method that uses an aqueous two-phase system for the isolation of ER is also described (10). Those interested in these details for these methods should consult the original articles. 

It is well-established that marker enzyme activity is an index of organelle (or organelle-related component) existence. In this experiment, assays of marker enzymes, which are localized in organelle membranes, are carried out as described below. 

Membrane transport in organelles, MEMBRANE TRANSPORT Menadione reductase,... 

Naringenin, other flavones, and the cinnamic acids could behave like quercetin. Hence, at least some of the phenolic allelochemlcals could prevent the utilization of ATP energy required for transport of materials across cellular membranes by inhibiting the hydrolysis of ATP. Conceivably, alterations induced to the permeability of organelle membranes coupled... 

Haloalkanes. Certain haloalkanes and haloalkane-containing mixtures have been demonstrated to potentiate carbon tetrachloride hepatotoxicity. Pretreatment of rats with trichloroethylene (TCE) enhanced carbon tetrachloride-induced hepatotoxicity, and a mixture of nontoxic doses of TCE and carbon tetrachloride elicited moderate to severe liver injury (Pessayre et al. 1982). The researchers believed that the interaction was mediated by TCE itself rather than its metabolites. TCE can also potentiate hepatic damage produced by low (10 ppm) concentrations of carbon tetrachloride in ethanol pretreated rats (Ikatsu and Nakajima 1992). Acetone was a more potent potentiator of carbon tetrachloride hepatotoxicity than was TCE, and acetone pretreatment also enhanced the hepatotoxic response of rats to a TCE-carbon tetrachloride mixture (Charbonneau et al. 1986). The potentiating action of acetone may involve not only increased metabolic activation of TCE and/or carbon tetrachloride, but also possible alteration of the integrity of organelle membranes. Carbon tetrachloride-induced liver necrosis and lipid peroxidation in the rat has been reported to be potentiated by 1,2- dichloroethane in an interaction that does not involve depletion of reduced liver glutathione, and that is prevented by vitamin E (Aragno et al. 1992). 

Lipoproteins or glycoproteins are the macromolecules that most commonly form receptors. They are often firmly embedded in the plasma membrane or cell-organelle membrane as intrinsic proteins (see section 7.1). At times, this renders their isolation and subsequent functional reconstitution difficult, as their structure may be dependent upon the surrounding membrane. Isolation of such a receptor molecule may cause its structural collapse, even to the extent that specific binding properties are lost. 

FIGURE 11-2 Lipid composition of the plasma membrane and organelle membranes of a rat hepatocyte. The functional specialization of each membrane type is reflected in its unique lipid composition. Cholesterol is prominent in plasma membranes but barely detectable... 

When one component of the pool is temporarily depleted, a new equilibrium is quickly established to replenish it. Movement of the sugar phosphates between intracellular compartments is limited specific transporters must be present in an organelle membrane. 

After synthesis on the smooth ER, the polar lipids, including the glycerophospholipids, sphingolipids, and glycolipids, are inserted into specific cellular membranes in specific proportions, by mechanisms not yet understood. Membrane lipids are insoluble in water, so they cannot simply diffuse from their point of synthesis (the ER) to their point of insertion. Instead, they are delivered in membrane vesicles that bud from the Golgi complex then move to and fuse with the target membrane (see Fig. 11-23). Cytosolic proteins also bind phospholipids and sterols and transport them between cellular membranes. These mechanisms contribute to the establishment of the characteristic lipid compositions of organelle membranes (see Fig. 11-2). 

Site of electron trans- Cell membrane Organelle membranes... 

The a helix is an important component of integral membrane proteins. These are proteins that traverse the hydrophobic plasma and organelle membranes (see Chapter 9) and perform important biologic functions. The portion of the protein that is embedded in the membrane is a-helical because the a helix provides for a maximum number of hydrogen bonds, which serve to reduce the hydrophilic nature of peptide linkages. The side chains of such trans-membrane a helices are also hydrophobic, even though under normal circumstances, such amino acids would prefer to form other secondary structures. 

Although proteins are large molecules they are small compared with a cell and even with supramolecular structures which may be part of a cell, such as plasma and organelle membranes, ribosomes, chromosomes, filaments, enzyme complexes and viruses (Chap. 1). Supramolecular structures are also prominent outside cells and are, for example, essential components of connective tissues such as tendon, ligament, cartilage and bone. Supramolecular structures can consist of a variety of different types of molecule from the small (such as membrane lipids) to macromolecules (such as proteins, DNA and RNA). 

In subsequent discussions, other examples will be given which support the view that allelopathic interference with crops may operate through effects on water balance and mineral nutrition. A number of allelopathic chemicals other than phenolic acids may also have their initial effects on cellular and organelle membranes. A disruption of membranes not only affects nutrient transport, but respiratory coupling, photosynthesis reactions, and other membrane associated processes. 

The plasma membrane is a major barrier to the diffusion of solutes into and out of plant cells, the organelle membranes play an analogous role for the various subcellular compartments, and the tonoplast performs this function for the central vacuole. For instance, although H20 and C02 readily penetrate the plasma membrane, ATP and metabolic intermediates usually do not diffuse across it easily. Before we mathematically describe the penetration of membranes by solutes, we will briefly review certain features of the structure of membranes. 

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