Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UNIPROT:Q8NEX9 (reductase)
 26,410 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

A chimeric gene consisting of the coding sequence for the membrane domain of the endoplasmic reticulum protein, 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, fused to the coding sequence for the soluble enzyme, beta-galactosidase of Escherichia coli, has been previously constructed. This fusion protein, HMGal, has been localized to the membrane of the endoplasmic reticulum of Chinese hamster ovary cells transfected with this chimeric gene, and its beta-galactosidase activity has declined in the presence of low density lipoprotein (Skalnik, D. G., Narita, H., Kent, C., and Simoni, R. D. (1988) J. Biol. Chem. 263, 6836-6841). In this report, we demonstrate that the loss of beta-galactosidase activity results from the accelerated degradation of the HMGal protein. Taking advantage of a fluorescence-activated cell sorter technique, we have selected transfected cells which express sufficient levels of HMGal to improve its immunodetection. Based on pulse-chase experiments, the half-life of HMGal is 6.0 h, and, in the presence of 20 mM mevalonate, the half-life declines 1.7-fold. Under these conditions, mevalonate accelerates the degradation of HMG-CoA reductase in these cells 1.6-fold, from 8.4 h to 5.3 h, most probably by the same mechanism. This mevalonate-regulated degradation of HMGal is not due to a heteromeric association of HMGal with reductase, since the same effect has been observed in cells lacking the reductase protein. In addition, we demonstrate that inhibition of protein synthesis with cycloheximide abolishes the mevalonate-dependent accelerated degradation of HMGal, in agreement with previous studies which have presented indirect evidence that a short-lived protein is essential for mediating the loss of HMG-CoA reductase activity. Finally, using brefeldin A, we show that the mevalonate-dependent accelerated degradation of HMGal may occur in the endoplasmic reticulum.
 ...
PMID:The regulated degradation of 3-hydroxy-3-methylglutaryl-CoA reductase requires a short-lived protein and occurs in the endoplasmic reticulum. 225 43

We examined the effects of a prolonged treatment with lovastatin, a potent competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, on the morphology and function of rat Leydig cells. Twenty-four h after the first lovastatin injection, no conspicuous ultrastructural changes were found, but isolated Leydig cells showed a notable reduction in their basal and HCG-stimulated testosterone production. By prolonging lovastatin administration (daily injections for 3 and 5 days), Leydig cells progressively recovered their secretory activity, and this was associated with a striking proliferation of smooth endoplasmic reticulum and peroxisomes. The hypothesis is discussed that these morphologic changes are the counterpart of an enhanced newly synthesis of HMG-CoA reductase, that is the expression of a compensatory response of Leydig cells aimed at maintaining an adequate production of cholesterol (i.e. testosterone precursors) in spite of the chronic competitive inhibition of HMG-CoA reductase by lovastatin.
 ...
PMID:Effects of prolonged administration of lovastatin, an inhibitor of cholesterol synthesis, on the morphology and function of rat Leydig cells. 227 22

NADH-cytochrome b5 reductase is known to be located on two distinct membranes, i.e. endoplasmic reticulum and outer mitochondrial membranes. The endoplasmic-reticulum-associated form of the enzyme contains myristic acid in an amide linkage to its N-terminal glycine [Ozols, Carr & Strittmatter (1984) J. Biol. Chem. 259, 13349-13354]. To investigate whether the dual subcellular localization of the reductase corresponds to a difference in fatty acylation, the enzyme was purified from well-characterized rat liver microsomal and mitochondrial fractions and analysed by a new quantitative analytical procedure. The purified reductases were run on SDS/polyacrylamide gels and blotted on to polyvinylidene difluoride membranes. The reductase-containing bands were treated with hydroxylamine, and amide-linked fatty acids were then detached by acid hydrolysis. The detached fatty acids were extracted, derivatized and analysed as phenylacyl esters by reverse-phase h.p.l.c., and the protein content of the samples was determined by amino acid analysis of the acid hydrolysates. Myristic acid was found in both the microsomal and mitochondrial reductases in a molar ratio of 1:1 with protein. These results demonstrate for the first time the presence of a myristylated protein on outer mitochondrial membranes, and show that the microsomal and mitochondrial reductases are also identical in their fatty acylation.
 ...
PMID:Both the outer mitochondrial membrane and the microsomal forms of cytochrome b5 reductase contain covalently bound myristic acid. Quantitative analysis on the polyvinylidene difluoride-immobilized proteins. 231 90

The specific activity of antimycin A-insensitive nicotinamide adenine dinucleotide (NADH)-dependent cytochrome C reductase, an enzyme associated with endoplasmic reticulum, was determined in the superior temporal, entorhinal, and cerebellar cortex of 16 patients who died with Alzheimer's disease and eight nondemented controls. The specific activity of choline acetyltransferase was also measured to provide an index of presynaptic cholinergic dysfunction. Our results revealed reciprocal changes in these activities that were of similar magnitude across the three regions examined. Furthermore, cytochrome C reductase activity was positively correlated with the density of neurofibrillary tangles, especially in the superior temporal cortex. These results support the hypothesis that Alzheimer's disease may be associated with an alteration of endoplasmic reticulum and the functions related to this intracellular membrane system, including the post-translational modification and localization of essential proteins.
 ...
PMID:Brain regional analysis of NADH-cytochrome C reductase activity in Alzheimer's disease. 233 81

Both 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase isozymes of the yeast Saccharomyces cerevisiae are predicted to contain seven membrane-spanning domains. Previous work had established the utility of the histidinol dehydrogenase protein domain, encoded by HIS4C, as a topologically sensitive monitor that can be used to distinguish between the lumen of the endoplasmic reticulum and the cytoplasm. This study directly tested the structural predictions for HMG-CoA reductase by fusing the HIS4C domain to specific sites in the HMG-CoA reductase isozymes. Yeast cells containing the HMG-CoA reductase-histidinol dehydrogenase fusion proteins grew on histidinol-containing medium if the HIS4C domain was present on the cytoplasmic side of the endoplasmic reticulum membrane but not if the HIS4C domain was targeted to the endoplasmic reticulum lumen. Systematic exchanges of transmembrane domains between the isozymes confirmed that both isozymes had equivalent membrane topologies. In general, deletion of an even number of putative transmembrane domains did not interfere with the topology of the protein, but deletion or duplication of an odd number of transmembrane domains inverted the orientation of the protein. The data confirmed the earlier proposed topology for yeast HMG-CoA reductase, demonstrated that the yeast enzymes are core glycosylated, and provided in vivo evidence that the properties of transmembrane domains were, in part, dependent upon their context within the protein.
 ...
PMID:Genetic and biochemical evaluation of eucaryotic membrane protein topology: multiple transmembrane domains of Saccharomyces cerevisiae 3-hydroxy-3-methylglutaryl coenzyme A reductase. 240 52

3-Hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase is the limiting enzyme step in cholesterol formation in mammalian liver and other tissues. It is a glycoprotein of 97,000 daltons embedded in the endoplasmic reticulum with a long cytoplasmic extension that is the site of catalytic conversion of HMG CoA to mevalonate. The enzyme is subject to both long-term (induction/repression; degradation) and short-term control (reversible phosphorylation) mediated by endocrine signaling (insulin, glucagon) and through negative feedback by metabolic products of mevalonate (e.g., cholesterol). The catalytic capacity of microsomal reductase falls rapidly in the presence of several protein kinases (reductase kinase, protein kinase-C, calmodulin-dependent protein kinase). Activity is restored with various protein phosphatases. Increased phosphorylation of reductase in intact cells after addition of glucagon or mevalonate is followed by enhanced degradation of the enzyme. In an in vitro model system, phosphorylated, native microsomal reductase is more rapidly cleaved by the calcium-dependent, neutral protease calpain than the dephosphorylated from of reductase. Our present research which centers on the mechanism of the in vitro model system is reviewed. Calpain in the presence of Ca2+ cleaves the cytosolic domain of phosphorylated 97 kDa reductase at two points giving rise to two fragments of nearly the same size that appear as a 52-56,000 dalton doublet by electrophoresis and immunoblotting. In the same system native reductase labeled with [gamma-32P]ATP generates a doublet with 32P solely in the upper (heavier) band. This indicates that serine phosphorylation sites lie between the two calpain cleavage loci. These are positioned in the "linker" region of the long carboxy-terminal cytosolic domain near the membrane. This segment possesses five invariant serine residues and two PEST sequences (constellations of proline, glutamate, serine and threonine) that are characteristic of proteins with short half-lives. If phosphorylation of HMG CoA reductase is confined to the linker region, we must look to this domain in order to interpret the resulting conformational changes that markedly influence reductase catalytic activity and prepare the enzyme for degradation.
 ...
PMID:Phosphorylation and degradation of HMG CoA reductase. 262 76

A ketone reducing enzyme was purified to homogeneity from female mouse liver microsomes, using the diagnostic cytochrome P-450 inhibitor metyrapone as a substrate. In contrast to the usually employed indirect spectrophotometric recording of pyridine nucleotide oxidation at 340 nm, a HPLC method was applied for direct alcohol metabolite determination. Purification of the carbonyl reductase resulted in a 360-fold increase in specific activity together with a single band in the 34 kD region after SDS-polyacrylamide gel electrophoresis. Phenobarbital, indomethacin, dicoumarol and 5 alpha-dihydrotestosterone inhibited the enzyme, whereas quercitrin did not affect the enzyme activity. Thus, by inhibitor classification of carbonyl reductases the ketone metyrapone is reduced by an aldehyde reductase, rather than by a ketone reductase. Dihydrotestosterone, the strongest inhibitor, is supposed to be the physiological substrate for the purified enzyme. It was demonstrated that during the steps of purification both NADPH and NADH can supply the required reducing equivalents, although the activity with NADH is weaker. The highest activity was obtained using an NADPH-regenerating system. Ethanol and the nonionic detergent Emulgen 913 led to an increased specific activity, indicating that the enzyme is bound to the membranes of the endoplasmic reticulum in a latent state. From these results it is concluded that the microsomal metyrapone-reducing enzyme belongs to the family of carbonyl reductases, but differs from the common patterns of their classification with regard to cofactor requirement and inhibitor susceptibility.
 ...
PMID:Purification and properties of a metyrapone-reducing enzyme from mouse liver microsomes--this ketone is reduced by an aldehyde reductase. 267 47

Change in the activity of cytoplasmic 5 beta- and microsomal 5 alpha-steroid reductases was studied in tumor bearing rats. As shown by thin-layer chromatography conversion of 3H-corticosterone into 5-alpha and 5 beta-dihydro-compounds was decreased 3-fold in rats bearing Walker tumor. Activity of both these enzymes exceeded the initial level 2-3-fold in presence of exogenous NADPH. At the same time, activation of glucose 6-phosphate dehydrogenase was accompanied by simultaneous alteration in content of transcortin-bound corticosterone in rat blood plasma. 5 alpha-reductase was detected in membranes of liver tissue endoplasmic reticulum and in the fraction of free polyribosomes.
 ...
PMID:[Activity of 5-alpha- and 5-beta-steroid reductases in subcellular liver structures from rats bearing Walker carcinosarcoma]. 274

Previous evidence suggests that the increase in platelet membrane fluidity associated with a subgroup of patients with Alzheimer's disease results from the accumulation of internal membrane. The specific activities of enzyme markers for selective cell membrane compartments were compared in platelets from subgroups of demented patients with normal or increased fluidity as well as from normal control subjects. A statistically significant change in enzyme activity was observed only for antimycin A-insensitive NADH-cytochrome reductase, a selective marker for smooth endoplasmic reticulum (SER) in platelets. This reduction was limited to the subgroup of demented patients who had increased platelet membrane fluidity, and therefore is not a nonspecific concomitant of neurodegeneration, medication exposure, or chronic illness in general. Since the platelet membrane alteration associated with Alzheimer's disease results from the inheritance of a single major locus, these results suggest that a defect in SER function may exist in brain cells as well as peripheral cells that express this genotype.
 ...
PMID:Endoplasmic reticulum abnormality in Alzheimer's disease: selective alteration in platelet NADH-cytochrome c reductase activity. 274 32

These studies test the hypothesis that a major determinant of excessive biliary cholesterol secretion is a level of hepatic sterol synthesis that is inappropriately high relative to the needs of the liver cell for preserving cholesterol balance. Biliary cholesterol secretion was measured in vivo in two models after loading the hepatocyte with sterol by two different mechanisms. In the first model, cholesterol was delivered physiologically to the liver in chylomicron remnants. This resulted in a sixfold increase in cholesteryl ester content and marked suppression of cholesterol synthesis, but biliary cholesterol secretion remained essentially constant. In the second model, 3-hydroxy-3-methyl-glutaryl CoA reductase levels in the liver were markedly increased by chronic mevinolin (lovastatin) administration. Withdrawal of the inhibitor resulted in a sudden fivefold increase in the rate of sterol synthesis in the liver of the experimental animals that was inappropriately high for cellular needs. This excessive synthesis, in turn, was accompanied by a fivefold increase in the cholesteryl ester content, enrichment of microsomal membranes with cholesterol and, most importantly, by a threefold increase in the rate of biliary sterol secretion. As the rate of sterol synthesis gradually returned to normal over 48 h, the cholesterol ester content, the lipid composition of the microsomal membranes, and rate of cholesterol secretion into bile also returned to baseline values. These results further support the concept of functional compartmentalization of cholesterol in the hepatocyte. Derangements that cause an inappropriately high rate of sterol synthesis in the endoplasmic reticulum may lead to an expansion of that pool of cholesterol that is recruitable by bile acids and, hence, to greater situation of the bile.
 ...
PMID:Inappropriate hepatic cholesterol synthesis expands the cellular pool of sterol available for recruitment by bile acids in the rat. 279 55


<< Previous 1 2 3 4 5 6 7 8 9 10 Next >>