Subcellular cytoplasm

In addition to effects on biochemical reactions, the inhibitors may influence the permeability of the various cellular membranes and through physical and chemical effects may alter the structure of other subcellular structures such as proteins, nucleic acid, and spindle fibers. Unfortunately, few definite examples can be listed. The action of colchicine and podophyllin in interfering with cell division is well known. The effect of various lactones (coumarin, parasorbic acid, and protoanemonin) on mitotic activity was discussed above. Disturbances to cytoplasmic and vacuolar structure, and the morphology of mitochondria imposed by protoanemonin, were also mentioned. Interference with protein configuration and loss of biological activity was attributed to incorporation of azetidine-2-carboxylic acid into mung bean protein in place of proline. 

Penninks Seinen (1980) looked at subcellular distribution of dibutyltin in rat liver and thymus cells in vitro. Radioactivity was concentrated in mitochondria and low in cytoplasm in thymus cells, in marked contrast to liver cells, where mitochondrial radioactivity was very low. Differences in cellular distribution have been suggested as a reason for the selective effect on the thymus. 

For the sake of study, the biosynthesis of carotenoid plant pigments can be divided into parts involving enzymes and their associated activities as listed in Table 5.3.1 and further detailed in Figure 5.3.1 through Figure 5.3.4. Some of the parts have common enzymatic mechanisms and may also be in distinct subcellular compartments such as cytoplasm, endoplasmic reticulum, or plastid thylakoid. 

Poly(3HB) synthesis in various subcellular compartments could be used to study how plants adjust their metabolism and gene expression to accommodate the production of a new sink, and how carbon flux through one pathway can affect carbon flux through another. For example, one could study how modifying the flux of carbon to starch or lipid biosynthesis in the plastid affects the flux of carbon to acetyl-CoA and poly(3HB). Alternatively, one could study how plants adjust the activity of genes and proteins involved in isoprenoid and flavonoid biosynthesis to the creation of the poly(3HB) biosynthetic pathway in the cytoplasm, since these three pathways compete for the same building block, i. e., acetyl-CoA. 

Lipids are transported between membranes. As indicated above, lipids are often biosynthesized in one intracellular membrane and must be transported to other intracellular compartments for membrane biogenesis. Because lipids are insoluble in water, special mechanisms must exist for the inter- and intracellular transport of membrane lipids. Vesicular trafficking, cytoplasmic transfer-exchange proteins and direct transfer across membrane contacts can transport lipids from one membrane to another. The best understood of such mechanisms is vesicular transport, wherein the lipid molecules are sorted into membrane vesicles that bud out from the donor membrane and travel to and then fuse with the recipient membrane. The well characterized transport of plasma cholesterol into cells via receptor-mediated endocytosis is a useful model of this type of lipid transport. [9, 20]. A brain specific transporter for cholesterol has been identified (see Chapter 5). It is believed that transport of cholesterol from the endoplasmic reticulum to other membranes and of glycolipids from the Golgi bodies to the plasma membrane is mediated by similar mechanisms. The transport of phosphoglycerides is less clearly understood. Recent evidence suggests that net phospholipid movement between subcellular membranes may occur via specialized zones of apposition, as characterized for transfer of PtdSer between mitochondria and the endoplasmic reticulum [21]. 

The cellular and subcellular distributions of a-subunit isoforms provide clues to their different physiological functions. The four isoforms exhibit about 85% sequence identity. The most substantial differences occur in their N-terminal regions and in an 11-residue sequence of the large cytoplasmic loop. When measured in cell cultures, the isoforms differ in their apparent affinities for intracellular Na+ (al < a2 < a3) [21 ] and extracellular K+ (a3 < a2 = al) [22], In adult tissues, al is the major iso form in... 

Histamine synthesis in the brain is controlled by the availability of L-histidine and the activity of histidine decarboxylase. Although histamine is present in plasma, it does not penetrate the blood-brain barrier, such that histamine concentrations in the brain must be maintained by synthesis. With a value of 0.1 mmol/1 for L-histidine under physiological conditions, HDC is not saturated by histidine concentrations in the brain, an observation that explains the effectiveness of large systemic doses of this amino acid in raising the concentrations of histamine in the brain. The essential amino acid L-histidine is transported into the brain by a saturable, energy-dependent mechanism [5]. Subcellular fractionation studies show HDC to be localized in cytoplasmic fractions of isolated nerve terminals, i.e. synaptosomes. 

Also, RU 58668 modifies the subcellular distribution of ER, appearing as clusters in the perinuclear region of cytoplasm, without association to specific cellular structures. This means that after RU 58668 treatment, ER is sequestered in the cytoplasm associated to short half-life proteins (probably induced by RU 58668 treatment) that impede its entry into the nucleus (Devin-Leclerc et al. 1998). 

Both vacuoles and cytoplasm can be visualized by ester-loading imper-meant fluorochromes. The fluorochrome 6CF can be introduced by ester-loading with 6CF-diacetate, which is not fluorescent or polar and readily permeates cells. Once inside the cell, it is cleaved into the highly fluorescent and charged anion 6CF, which is ion-trapped. Lipophilic FITC derivatives are compartmented in patterns that depend on the subcellular location of esterases, and in different cells may be compartmented by cytoplasm or vacuole (51). 

The intracellular environment of eukaryote cells can be subdivided into many regions, including the organelles, nucleus, cytoplasm and the cell periphery. Thus solutes must be delivered to the right intracellular compartment at the correct time to efficiently serve cellular biochemistry. Uncharged solutes such as glucose presumably diffuse across the cell, and the traditional view held until recently was that the major electrolytes, such as Na+,K+,CF and Mg2+, also move around the cell by simple diffusion to eventually arrive at the relevant subcellular compartment by chance. 

Subcellular structures (organelles) are present in the cell. Each one has its own characteristic activities and properties that work together to maintain the cell and its functions. The remainder of the cell is the gel-like cytoplasm, known as cytosol (Eigure 1.1). The largest organelle in the cell is the nucleus it contains the genetic... 

While the abundance of DBT does not oscillate, the subcellular localization of DBT does change throughout the circadian day (Fig. 2). In Drosophila, the subcellular distribution of DBT in the lateral neurons of the brain and in photoreceptor cells of the eye largely follows the changing localization of PER (Kloss et al 2001). In mammals, the pattern of CKl accumulation is also under circadian control the kinase appears to associate initially with mPER and mCRY in the cytoplasm, but it is also found in nuclear complexes and may regulate their movement to the nucleus (Lee et al 2001). 

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