Endoplasmic Reticulum: A Metabolic Compartment
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The endoplasmic reticulum is a continuous membrane network in the cytosol, which encloses its internal compartment, the endoplasmic reticulum lumen. Several metabolic pathways are compartmentalised within the ER lumen, for example hydrolysis of glucose 6-phosphate, glucuronidation of endo- xenobiotics, posttranslational modification of proteins including redox reactions required for oxidative folding, oxidoreduction of steroid hormones, synthesis of ascorbate. Therefore, enzyme activities of these pathways depend on the special luminal microenvironment, on access to substrates and on release of products. However, in spite of great efforts, the molecular mechanism for the generation and maintenance of this special microenvironment still remains to be elucidated. Beside the well-known functions of the endoplasmic reticulum, such as calcium signaling and the synthesis of secretory proteins, recent findings underlined the importance of the intraluminal redox biochemistry and the role of membrane transporters. The field is currently undergoing extensive reappraisal, new transporters have been identified either molecular or functional level. The local synthesis and the membrane transport of redox active compounds (pro- and antioxidants) seem to be important not only in the disulfide bond formation, but also in the generation of intracellular proliferative/apoptotic signals. The different points of views in this publication help highlight the potential importance of several recently described phenomena, whose significance needs elucidation.