Cytosol enzymes

Cytosolic enzyme constitutively present mainly in nervous tissues and activated in the presence of Ca2+ and calmodulin. 

Maretski, D., Reimann, B., Rapoport, S.M. (1989). A reappraisal of the binding of cytosolic enzymes to erythrocyte membranes. Trends Biochem. Sci. 14,93-96. 

The only other examples of bromoconduritol inhibition reported so far are a cytosolic jff-D-glucosidase from calf liver and the lysosomal ff-D-glu-cosidase from calf spleen. In spite of the 6500-fold difference in their reactivity with conduritol B epoxide (see Table XI), both enzymes are rapidly inactivated by bromoconduritol F, with kj(max)/Kj 10 M min for the cytosolic enzyme and lq(max)/Ki 3.2 10 for the crude and 3.9 10 M min for the purified lysosomal enzyme. It should be noted that purification of the lysosomal jS-D-glucosidase had effects on the reactivity with bromoconduritol F similar to those it had on the reactivity with conduritol B epoxide (see Table XI). 

The red cell contains a battery of cytosolic enzymes, such as superoxide dismutase, catalase, and glutathione peroxidase, to dispose of powerful oxidants generated during its metabolism. 

Carotene cleavage enzymes — Two pathways have been described for P-carotene conversion to vitamin A (central and eccentric cleavage pathways) and reviewed recently. The major pathway is the central cleavage catalyzed by a cytosolic enzyme, p-carotene 15,15-oxygenase (BCO EC 1.13.1.21 or EC 1.14.99.36), which cleaves p-carotene at its central double bond (15,15 ) to form retinal. Two enzymatic mechanisms have been proposed (1) a dioxygenase reaction (EC 1.13.11.21) that requires O2 and yields a dioxetane as an intermediate and (2) a monooxygenase reaction (EC 1.14.99.36) that requires two oxygen atoms from two different sources (O2 and H2O) and yields an epoxide as an intermediate. ... 

Phoenix, J., Edwards, R.H.T. and Jackson, M.J. (1989). Inhibition of calcium-induced cytosolic enzyme efflux from skeletal muscle by vitamin E and related compounds. Biochem. J. 287, 207-213. 

The intestinal mucosal peptidases are distributed in the brush border and cytosol of the absorptive cell. There are, however, distinct differences between the brush border and cytosolic peptidases [75], The tetrapeptidase activity is associated exclusively with the brush border enzyme. Furthermore, brush border peptidases exhibit more activity against tripeptides than dipeptides, whereas the cytosolic enzymes show greater activity against dipeptides. Studies have demonstrated that more than 50% of dipeptidase activity was detected in the cytosol [76] and just 10% in the brush border membrane [77]. The brush border enzymes include... 

C Doumeng, S Maroux. Aminotripeptidase, a cytosol enzyme from rabbit intestinal mucosa. Biochem J 177 801-808, 1979. 

Sulfotransferases are cytosolic enzymes that add sulfate to phenolic and hydroxyl functions as well as certain amines. The relative rates of sulfation of several sulfo-... 

The majority of CYP enzymes are located in a hydrophobic environment in the endoplasmic reticulum of cells, although cytosolic enzymes also exist, such as CYP101. In order to mimic the physiological environment of CYP enzymes, a number of groups have used phospholipids to construct biosensors such as DDAB, dimeristoyl-L-a-phosphatidylcholine (DMPC), dilauroylphosphatidylethanolamine (DLPE) and distearoylphosphatidylethanolamine (DSPE). Phospholipid layers form stable vesicular dispersions that bear structural relationship with the phospholipid components of biologically important membranes. By this way a membranous environment is created that facilitates electron transfer between the enzyme s redox center and the electrode. 

Xanthine oxidoreductase (XOR) is a molybdenum-containing complex homodimeric 300-kDa cytosolic enzyme. Each subunit contains a molybdopterin cofactor, two nonidentical iron-sulfur centers, and FAD (89). The enzyme has an important physiologic role in the oxidative metabolism of purines, e.g., it catalyzes the sequence of reactions that convert hypoxanthine to xanthine then to uric acid (Fig. 4.36). 

The intracellular localization of carboxylesterases is predominantly microsomal, the esterases being localized in the endoplasmic reticulum [73] [79] [93], They are either free in the lumen or loosely bound to the inner aspect of the membrane. The carboxylesterases in liver mitochondria are essentially identical to those of the microsomal fraction. In contrast, carboxylesterases of liver lysosomes are different, their isoelectric point being in the acidic range. Carboxylesterase activity is also found in the cytosolic fraction of liver and kidney. It has been suggested that cytosolic carboxylesterases are mere contaminants of the microsomal enzymes, but there is evidence that soluble esterases do not necessarily originate from the endoplasmic reticulum [94], In guinea pig liver, a specific cytosolic esterase has been identified that is capable of hydrolyzing acetylsalicylate and that differs from the microsomal enzyme. Also, microsomal and cytosolic enzymes have different electrophoretic properties [77]. Cytosolic and microsomal esterases in rat small intestinal mucosa are clearly different enzymes, since they hydrolyze rac-oxazepam acetate with opposite enantioselectivity [95], Consequently, studies of hydrolysis in hepatocytes reflect more closely the in vivo hepatic hydrolysis than subcellular fractions, since cytosolic and microsomal esterases can act in parallel. 

The cytosolic enzyme leukotriene A4 hydrolase (EC 3.3.2.6), which ster-eoselectively converts leukotriene A4 (LTA4) to leukotriene B4 [56], This enzyme catalyzes the hydrolytic cleavage of the 5,6-epoxide ring in LTA4, but, in contrast to what happens with other EHs, the product is not a vicinal diol but a 5,12-diol. As a zinc metalloenzyme, LTA4 hydrolase does not appear to be related to any other epoxide hydrolase. 

Note. The supernatant, the raw cytosohc fraction, could be saved for another experiment as a source of liver cytosolic enzymes. 

Moreno SNJ, Mason RP, Docampo R. 1984. Distinct reduction of nittrofurans and metronidazole to free radical metabolites by Tritrichomonas foetus hydrogenosomal and cytosolic enzymes. J Biol Chem 259 8252-9. 

LO activity is found in a variety of mammalian cells, most notably leukocytes, with the enzymes from neutrophils and the rat basophilic leukemia (RBL-1) cell line being the most studied several have been purified and cloned [5]. 5-LO is a cytosolic enzyme which requires calcium, ATP and additional uncharacterized cell components for optimal activity. Characterization and kinetic studies with 5-LO enzymes are often difficult, because of self-inactivation and complex cofactor requirements. 

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