Cytosolic compartment

The final reactions to be considered in the metabolism of ethanol in the liver are those involved in reoxidation of cytosolic NADH and in the reduction of NADP. The latter is achieved by the pentose phosphate pathway which has a high capacity in the liver (Chapter 6). The cytosolic NADH is reoxidised mainly by the mitochondrial electron transfer system, which means that substrate shuttles must be used to transport the hydrogen atoms into the mitochondria. The malate/aspartate is the main shuttle involved. Under some conditions, the rate of transfer of hydrogen atoms by the shuttle is less than the rate of NADH generation so that the redox state in the cytosolic compartment of the liver becomes highly reduced and the concentration of NAD severely decreased. This limits the rate of ethanol oxidation by alcohol dehydrogenase. 

Figure 1. Hypothetical mechanism for shuttling of intermediates of the common aromatic pathway between plastidic and cytosolic compartments. Enzymes denoted with an asterisk (DAHP synthase-Co, chorismate mutase-2, and cytosolic anthranilate synthase) have been demonstrated to be isozymes located in the cytosol. DAHP molecules from the cytosol are shown to be shuttled into the plastid compartment in exchange for EPSP molecules synthesized within the plastid. Abbreviations C3, phosphoenolpyruvate C4, erythrose 4-P DAHP, 3-deoxy-D-arabino-heptulosonate 7-phosphate EPSP, 5-enolpyruvylshikimate 3-phosphate CHA, chorismate ANT, anthranilate TRP, L-tryptophan PPA, prephenate AGN, L-arogenate TYR, L-tyrosine and PHE, L-phenylalanine.

The tightly regulated pathway specifying aromatic amino acid biosynthesis within the plastid compartment implies maintenance of an amino acid pool to mediate regulation. Thus, we have concluded that loss to the cytoplasm of aromatic amino acids synthesized in the chloroplast compartment is unlikely (13). Yet a source of aromatic amino acids is needed in the cytosol to support protein synthesis. Furthermore, since the enzyme systems of the general phenylpropanoid pathway and its specialized branches of secondary metabolism are located in the cytosol (17), aromatic amino acids (especially L-phenylalanine) are also required in the cytosol as initial substrates for secondary metabolism. The simplest possibility would be that a second, complete pathway of aromatic amino acid biosynthesis exists in the cytosol. Ample precedent has been established for duplicate, major biochemical pathways (glycolysis and oxidative pentose phosphate cycle) of higher plants that are separated from one another in the plastid and cytosolic compartments (18). Evidence to support the hypothesis for a cytosolic pathway (1,13) and the various approaches underway to prove or disprove the dual-pathway hypothesis are summarized in this paper. 

N-Myristoylation is achieved by the covalent attachment of the 14-carbon saturated myristic acid (C14 0) to the N-terminal glycine residue of various proteins with formation of an irreversible amide bond (Table l). 10 This process is cotranslational and is catalyzed by a monomeric enzyme called jV-myri s toy 11ransferase. 24 Several proteins of diverse families, including tyrosine kinases of the Src family, the alanine-rich C kinase substrate (MARKS), the HIV Nef phosphoprotein, and the a-subunit of heterotrimeric G protein, carry a myr-istoylated N-terminal glycine residue which in some cases is in close proximity to a site that can be S-acylated with a fatty acid. Functional studies of these proteins have shown an important structural role for the myristoyl chain not only in terms of enhanced membrane affinity of the proteins, but also of stabilization of their three-dimensional structure in the cytosolic form. Once exposed, the myristoyl chain promotes membrane association of the protein. 5 The myristoyl moiety however, is not sufficiently hydrophobic to anchor the protein to the membrane permanently, 25,26 and in vivo this interaction is further modulated by a variety of switches that operate through covalent or noncovalent modifications of the protein. 4,5,27 In MARKS, for example, multiple phosphorylation of a positively charged domain moves the protein back to the cytosolic compartment due to the mutated electrostatic properties of the protein, a so-called myristoyl-electrostatic switch. 28 ... 

Collectively these studies suggest that although GSH and GSH-related enzymes are abundant in cytosolic compartments of astrocytes, the mitochondrial pools are... 

The half-life of M-MT is dependent on the binding affinity of thionein for different metal ions. For instance, upon oxidation, Cu-MT forms insoluble polymers which are biologically unavailable and are eventually eliminated via biliary secretion. In contrast, thionein has lower affinity for Zn, making it more easily released from the protein and rendering the ion available for cellular processes. Furthermore, the rate of degradation may be influenced by differences in metal distribution between MT isoforms. It has been determined that MT degradation can occur in lysosomal and nonlysosomal (cytosolic) compartments. 

The cytosolic compartment in mammalian cells is protected from oxidative damage caused by H2O2, by catalase and by the selenoenzyme glutathione peroxidase (GSHPx) GSHPx is also present in the mitochondria of all mammalian cells. 

Cova et al. [126] have recently investigated the intracellular distribution of PBN in rat myocardium. They found that a large fraction of PBN accumulates in the cytosolic compartment as compared to the nuclear and mitochondrial... 

The patterns of antigen distribution in different organelles are quite characteristic in cultured cells, and examples of such patterns have been published in many journal articles and books (e.g., ref. 2). From immunofluorescence images, antigens restricted to intracellular membranous organelles, cytoskeletal elements, cytosolic compartments, and nuclear... 

The uiea cycle may be considered to be a mitochondrial pathway, as carbamyl phosphate synthase and ornithine transcarbamylase are mitochondrial enzymes however, the enzymes catalyzing subsequent steps of the pathway arc cytosolic-The steps leading to conversion of citrulline to ornithine occur in the cytosol. Hence, the pathway is shared by the mitochondrial and cytosolic compartments. The fumarate produced by the urea cycle is converted to malate by a cytoplasmic form of fumarase. Mittxihondrial fumarase is part of the Krebs cycle. Cytoplasmic malate can enter the mitochondrion by means of a transport system, such as the malate/phosphate exchanger or the ma ate/a-ketoglutaratc exchanger. These transport systems are membrane-bound proteins. 

Fig. 3 Structural model of the cell membrane. The membrane is composed of a bimolecular leaflet of phospholipid with the polar head groups facing the extracellular and cytosolic compartments and the acyl groups in the middle of the bilayer. Integral membrane proteins are embedded in the lipid bilayer. Integral proteins are glycosylated on the exterior surface and may be phosphorylated on the cytoplasmic surface. Extrinsic membrane proteins, peripheral proteins, are linked to the cytosolic surface of the intrinsic proteins by electrostatic interactions. (From Ref. l)...

The pancreatic B-cells in general are rich in Ca2+. When calculated in terms of intracellular water, the concentration of Ca2+ in mouse B-cells ranges from 16 to 25 mM (Heilman, 1986 ). Conversely, Ca2+ in the cytosolic compartment which serves as a second messenger is only in the nanomolar range. 

Although it is well accepted that the synergistic interaction between DAG and calcium is responsible for protein kinase C activation, the biochemical mechanisms that mediate this activation in vivo are complex. Several studies have demonstrated that cellular activation is accompanied by the translocation of protein kinase C from the cytosolic compartment to cellular membranes (e.g., Ganong et al., 1986). Increases in cytosolic calcium promote protein kinase C translocation to cellular membranes, and removal of calcium accelerates the dissociation of protein kinase C from cellular membranes in a process that appears to be modulated by ATP (Bell, 1986). 

A transgenic mouse model expressing a PrP mutant without the N-terminal signal sequence (cytoPrP) conclusively showed that targeting of PrP to the cytosolic compartment can be neurotoxic [49, 68]. Further evidence for a toxic potential of cytosolically localized PrP was provided by additional mouse models [67, 69], by several mammalian cell culture models [49, 70, 71], and by a yeast model [72]. 

Different mechanisms have been described to explain the toxic effects of PrP in the cytosolic compartment. In one study co-aggregation of misfolded cytoPrP with the anti-apoptotic protein Bcl-2 was shown to coincide with toxicity [71]. Another study reported that cytoPrP can interact with the E3 ubiquitin ligase Mahogunin, thereby disrupting its function [67]. Interestingly, a toxic potential was also observed for cytosolically localized PrPSc, which can inhibit proteasomal activity [69]. 

These findings strongly support the notion that physiologically active PrPc is part of signaling cascades implicated in various cellular processes. Of note, GPI-anchored proteins like PrPc do not have direct contact with the cytosolic compartment and therefore require co-factors for intracellular signal transmission (reviewed in [115]). Attractive candidates for cellular components involved in PrPc-dependent signaling include NMDA receptors [106] and the intracellular tyrosine kinase Fyn [116, 117]. 

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