Cytosol, definition

There are cytosolic and membrane-bound isoforms of NO synthase. Certain soluble and particulate isoforms are constitutive and other soluble isoforms are inducible. The constitutive enzyme is, by definition, present in the catalytically active form and needs only to be stimulated by an appropriate chemical species, following which there is immediate formation of NO plus L-citrulline. This form of NO synthase requires calcium, and for the most part calmodulin, for stimulation of enzymatic activity. It is likely that an increase in intracellular free calcium in the presence of calmodulin is the signal for stimulation of NO synthase, and therefore, the production of NO. This view is consistent with the general understanding that, in vascular tissue, all endothelium-dependent vaso-... 

While isolation of a specific inhibitor will be necessary to assess the definitive role of the cytosolic nuclease in the low transfection efficiency in vivo, circumstantial evidence suggests that the metabolic instability of plasmid DNA represents one of the cellular barriers to gene transfer. Microinjection of DNA complexes with PEI has augmented the transfection efficiency (Pollard et al., 1998). Although the stability of the PEI-complexed DNA has not been determined in vivo, it has been demonstrated that the nuclease resistance of plasmid DNA is dramatically increased upon complex formation in vitro (Cappaccioli et al., 1993 Chiou et al., 1994 Thierry et al., 1997). Therefore, it is conceivable that faster diffusional mobility and decreased nuclease susceptibility jointly lead to the enhanced nuclear targeting efficiency of the PEI-condensed plasmid DNA. 

A second, cytosolic CPS activity (CPSII) occurs in mammals as part of the CAD trifunctional protein that catalyzes the first three steps of pyrimidine synthesis (CPSII, asparate tran-scarbamoylase, and dihydroorotase). The activities of these three enzymes—CPSII, aspartate transcarbamoylase, and dihydroorotase—result in the production of orotic acid from ammonium, bicarbonate, and ATP. CPSII has no role in ureagenesis, but orotic aciduria results from hepatocellular accumulation of carbamyl phosphate and helps distinguish CPSI deficiency from other UCDs. Defects in CPSI classically present with neonatal acute hyperammonemic encephalopathy. The plasma citrulline and urine orotic acid concentrations are both low. A definitive diagnosis can be established by enzyme assay of biopsied liver tissue or by mutation analysis. 

A low molecular weight protein, different from metallothionine which reversibly binds iron with high affinity has been isolated from rabbit reticulocyte cytosol (54, 55, 56). Although very little is yet known about its physiological properties, the molecular weight is around 6000, and iron appears to be reversibly bound under physiological conditions. This protein may be able to mobilize iron from the plasma membrane and donate it for heme and ferritin biosynthesis (56), but no definitive physiological role for siderochelin has been established. 

An intrinsic ionic charge gradient across the membrane exists because of semipermeable nature of membrane, which maintains a difference in the concentration of the ions between the cytosol and the extracellular matrix. This difference results in a definite potential across membrane of the normal cells, which is called the resting potential. Normal plant cells, mammalian muscle cells, and neurons have resting potential values of about —120, —90, and —70 mV, respectively. Along with the resistance to the flow of ions, membrane also exhibits a capacitance. Cm, which is given by... 

This definition has been the basis for an enormous body of scientific investigation into the function and regulation of receptors and mechanisms of drug action. However, final proof of the existence of receptors did not occur until relatively recent applications of modern biochemistry and molecular biology to purify, sequence, clone, and express pure receptor proteins. This lack of proof notwithstanding, the therapeutic basis of many modern, and not so modern, drugs resides in their specific interactions with receptor molecules located in the plasma membrane or cytosol of target cells. In fact, these specific interactions have provided the experimental basis for their discovery and development. 

The path of entry of extracellular Ca " during the sustained phase of contraction in ASM has eluded definition in the absence, to date, of the demonstration of physiologically relevant VDCs or ROCs. These data raise the possibility that following agonist activation, when the buffering capacity of the sr is abolished, the passive plasmalemmal leak can provide sufficient extracellular Ca " to sustain the onist-induced, or receptor-mediated (Murray and Kotlikoflf, 1991), plateau rise in cytosolic Ca which accompanies maintained contraction. 

Because an organelle is defined as a portion of a cell enclosed by a membrane, ribosomes are not, strictly speaking, organelles. Smooth endoplasmic reticulum does not have ribosomes attached, and ribosomes also occur free in the cytosol. The definition of organelle also affects discussion of the cell membrane, cytosol, and cytoskeleton. 

FIGURE 19.13 Transfer of the starting materials of lipid anabolism from the mitochondrion to the cytosol. (1 is ATP-citrate lyase other symbols are as in Figure 19.10.) It is not definitely established whether acetyl-CoA is transported from the mitochondrion to the cytosol. 

The enzymatic conjugation using rat liver cytosol produces a mixture of the 4- and 5-thioether conjugates. However, the diastereoisomers of the conjugates are not formed in equal amounts. With a purified enzyme from the Little River Skate, an equal mixture of the 4- and 5-thioether conjugates are produced. But in this case, only one of the diastereoisomers of each positional isomer is produced. Product analysis of the SH conjugates obtained from C-labeled BP-4,5-oxide (4,5- C) established some definite stereochemical requirements for the catalytic step (12). 

---