Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:3.4.23.5 (cathepsin D)
 4,130 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

We attempted to establish within which organelle UDP-Glc:ceramide beta 1----1'glucosyltransferase (GlcT) is located and moreover to obtain information about its orientation on intracellular membranes as well as that of UDP-Gal:glucosylceramide beta 1----4galactosyltransferase (GalT-2) and CMP-NeuAc:lactosylceramide alpha 2----3sialyltransferase (SAT-1). An extremely purified Golgi apparatus fraction was the only liver fraction where a ceramide-dependent formation of glucosylceramide could be demonstrated. This Golgi fraction, mainly constituted by stacks of intact cisternae which retained the same topographical orientation as in vivo, was then incubated with liposomal dispersions of glycosphingolipid-glycosyltransferase acceptors in reaction mixtures containing all the requirements for enzyme activity but no detergent. Under such conditions, SAT-1 and other late acting glycosyltransferases were over 90% latent, while both GlcT and GalT-2 were just as active as in the detergent-containing assay; they were still inhibited by EDTA. Sepharose-immobilized ceramide and Sepharose-immobilized glucosylceramide were found to be suitable acceptors for GlcT and GalT-2, respectively, still using intact Golgi cisternae as the enzyme source. Moreover, a part of GlcT and GalT-2 activity was released from intact Golgi cisternae upon cathepsin D treatment. These results provide strong evidence that GlcT and GalT-2 face the cytoplasmic side of the Golgi apparatus, whereas SAT-1 and the other late acting enzymes face the luminal side.
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PMID:Topography of glycosyltransferases involved in the initial glycosylations of gangliosides. 183 52

Golgi-membrane-bound Gal beta 1-4GlcNAc alpha 2-6-sialyltransferase (CMP-N-acetylneuraminate:beta-galactoside alpha 2-6-sialyltransferase, EC 2.4.99.1) behaves as an acute-phase reactant increasing about 5-fold in serum in rats suffering from inflammation. The mechanism of release from the Golgi membrane is not understood. In the present study it was found that sialyltransferase could be released from the membrane by treatment with ultrasonic vibration (sonication) followed by incubation at reduced pH. Maximum release occurred at pH 5.6, and membranes from inflamed rats released more enzyme than did membranes from controls. Galactosyltransferase (UDP-galactose:N-acetylglucosamine galactosyltransferase; EC 2.4.1.38), another Golgi-located enzyme, which does not behave as an acute-phase reactant, remained bound to the membranes under the same conditions. Release of the alpha 2-6-sialyltransferase from Golgi membranes was substantially inhibited by pepstatin A, a potent inhibitor of cathepsin D-like proteinases. Inhibition of release of the sialyltransferase also occurred after preincubation of sonicated Golgi membranes with antiserum raised against rat liver lysosomal cathepsin D. Addition of bovine spleen cathepsin D to incubation mixtures of sonicated Golgi membranes caused enhanced release of the sialyltransferase. Intact Golgi membranes were incubated at lowered pH in presence of pepstatin A to inhibit any proteinase activity at the cytosolic face; subsequent sonication showed that the sialyltransferase had been released, suggesting that the proteinase was active at the luminal face of the Golgi. Golgi membranes contained a low level of cathepsin D activity (EC 3.4.23.5); the enzyme was mainly membrane-bound, since it could only be released by extraction with Triton X-100 or incubation of sonicated Golgi membranes with 5 mM-mannose 6-phosphate. Immunoblot analysis showed that the transferase released from sonicated Golgi membranes at lowered pH had an apparent Mr of about 42,000 compared with one of about 49,000 for the membrane-bound enzyme. Values of Km for the bound and released enzyme activities were comparable and were similar to values reported previously for liver and serum enzymes. The work suggests that a major portion of sialyltransferase containing the catalytic site is released from a membrane anchor by a cathepsin D-like proteinase located at the luminal face of the Golgi and that this explains the acute-phase behaviour of this enzyme.
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PMID:The role of a cathepsin D-like activity in the release of Gal beta 1-4GlcNAc alpha 2-6-sialyltransferase from rat liver Golgi membranes during the acute-phase response. 314 77