Endoplasmic reticulum enzymes

Many of the phase 1 enzymes are located in hydrophobic membrane environments. In vertebrates, they are particularly associated with the endoplasmic reticulum of the liver, in keeping with their role in detoxication. Lipophilic xenobiotics are moved to the liver after absorption from the gut, notably in the hepatic portal system of mammals. Once absorbed into hepatocytes, they will diffuse, or be transported, to the hydrophobic endoplasmic reticulum. Within the endoplasmic reticulum, enzymes convert them to more polar metabolites, which tend to diffuse out of the membrane and into the cytosol. Either in the membrane, or more extensively in the cytosol, conjugases convert them into water-soluble conjugates that are ready for excretion. Phase 1 enzymes are located mainly in the endoplasmic reticulum, and phase 2 enzymes mainly in the cytosol. 

Formation of 7-hydroxycholesterol. The committed and rate-limiting reaction for bile acid synthesis is catalyzed by the endoplasmic reticulum enzyme 7c -hydroxylase. 

The P450 enzymes are found primarily in the outer membrane of the endoplasmic reticulum. Enzyme activity requires that the enzyme be integrated into a membrane that contains P450 reductase and, for some reactions, cytochrome b5. Characterization of the saturation kinetics for the P450 enzymes can be determined using a variety of enzyme preparations, including tissue slices, whole cells, microsomes, and reconstituted, purified enzymes. The more intact the in vitro preparation, the more it is likely that the environment of the enzyme will represent the in vivo environment. However, intact cell preparations do not... 

Lamande SR, Bateman JF Procollagen folding and assembly the role of endoplasmic reticulum enzymes and molecular chaperones. Semin Cell Dev Biol 10 455-464,1999. 

The desaturase catalyzing this final step in the synthesis of a plasmalogen is an endoplasmic reticulum enzyme akin to the one that introduces double bonds into long-chain fatty acyl CoA molecules. In both cases, O2 and NADH are reactants, and cytochrome b 5 participates in catalysis (Section 22.6). 

Cortisol biosynthesis by mitochondrial and endoplasmic reticulum enzymes of the adrenal cortex. The rate-limiting step in steroidogenesis is the importation of cholesterol from cytoplasm to mitochondria which is mediated by the steroidogenic acute regulatory protein (StAR). 

Testosterone biosynthetic pathways, (a) A" Pathway (preferred pathway in human testes), (b) A Pathway (all enzymes located in the endoplasmic reticulum). Enzymes 1,3/i-hydroxy.steroid dehydrogena.se and A-, A -i,somera.se 2, I7a-hydroxylase (CYP 17) 3, Ci7.2o-lyase 4, 17/3-hydroxysteroid dehydrogena.se. 

The last step in the synthesis of squalene is a reductive taii-to-tail condensation of two molecules of farnesyl pyrophosphate catalyzed by the endoplasmic reticulum enzyme squalene synthase. 

The answer is d. (Murray, pp 505-626. Scriver, pp 4029-4240. Sack, pp 121-138. Wilson, pp 287-320.) Although the liver is the major site of the formation of free glucose to maintain blood glucose levels, the kidneys and intestinal epithelium (e.g., duodenum, jejunum, and ileum) may also release glucose. All of these tissues contain the enzyme glucose-6-phosphatase, an endoplasmic reticulum enzyme that dephosphorylates glucose and allows it to be transferred out of the cells. No other tissues in mammals contain this enzyme. 

Beyond these various defense compoimds, plants synthesize a wide variety of signaling molecules (oxylipins, brassinosteroids, gibberel-lins, cytokinins, strictolactones) [25], pigments (chlorophylls, carotenoids) [26], and fatty acids and sterols [27]. In the perspective of this chapter, it is worth noting that the production of many of these plant compoimds depends on both chlo-roplast enzymes, which include a small number of soluble P450s, and endoplasmic reticulum enzymes, which include the bulk of membrane-bound P450s. 

The enzyme responsible for the final step of de novo biosynthesis of phosphatidylinositol (PI) is an endoplasmic reticulum enzyme the cytidine diphosphate-diacylglycerol (CDP-DAG) my< -inositol transferase (EC 2.7.8.11), or briefly PI-synthase. This enzyme catalyses the formation of PI and cytidine monophosphate (CMP) from CDP-DAG and myo-inosiiol [1], In a previous work [2], we found that the PI-synthase displayed a strong selectivity among the endogenous CDP-DAG molecular species used as substrates, in the microsomes from several plant tissues 16 0/18 2 and 16 0/18.3-CDP-DAG were markedly more utilized than other molecular species. Here, we are interested in the Pl-synthase selectivity among CDP-DAG molecular species, both when the enzyme was left within microsomal membranes or when it was solubilized and pre-purified. As a continuation of our work upon the solubilization of the Pl-synthase [3], we used CHAPS to solubilize the microsomal membranes as previously described. 

In summary, therefore, the rise of /3-glucuronidase activity (per se) measured in homogenate may reflect an increa.se in either endoplasmic reticulum enzyme, an increase in lysosomal enzyme, or in both. Before the interpretation of such an increase can be made exact, it is necessary to establish how much of the enzyme is coming from each subecllular location. 

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