Phospholipases degradation

The weak base chloroquine (30-300 pM, 30 minutes, either in the presence or absence of serum) increases the endosomal pH (83) within a few minutes (45), leading to pH values close to 6.3 in both endosomes and lysosomes and therefore preventing lysosomal degradation. As a mechanism of action, a direct inhibition of lysosomal hydrolases (cathepsin B1 and some phospholipases and lysophospholipases) is reported (82). In Kupffer cells, effects are tolerated at concentrations of 40 pM or less for up to four hours and are irreversible within two hours after medium replacement (82). 

In addition to DPIEL, other phosphatidyhnositol analogues (PIAs) (e.g., SH-5 and SH-6, compounds 36 and 37, respectively. Fig. 7) have been recently reported [ 159,160]. When used at 5 or 10 xM, SH-5 and SH-6, which are supposed to be more resistant to phosphatidyUnositol-specific phospholipase C mediated degradation [161], effectively block the phosphorylation and activation of PKB in HL60AR tumor cells, and sensitize this and other leukemic tumor cell hnes to the effects of etoposide and cytarabine. No effect on the survival rate of hematopoetic precursor cells was observed if the compounds are used at 5 xM. 

The intracellular signaltransduction of ofi-adrenoceptors is effectuated by a G-protein-dependent activation of the phospholipase C. This enzyme cleaves phosphatidylinositol, a phospholipid present in cell membranes, into inositol-1,4-5-triphosphate (IP3) and diacylglycerol (DAG). IP3 is a strong inductor of intracellular calcium release which leads to an increase of smooth muscle tone or the liberation of hormones stored in vesicles. Noradrenaline which is released by exocytosis, spreads by diffusion only. Only a small fraction of the total amount of the transmitter released will actually reach the postsynaptic membrane and bind to its specific receptors. Another fraction escapes the synapic cleft by diffusion and is finally enzymatically degraded in the interstitial fluid. Another fraction is taken up postsynaptically and metabolized enzymatically by the target cells (uptake 2). By far most of the transmitter (90%) is actively taken up by the releasing neuron itself (uptake 1 or neuronal re-uptake). In the... 

Expression The sustained high calcium concentrations pave the way for destructive cascades causing neuronal degeneration. Phospholipase, proteases, and FRs are activated, degrading neuronal membranes and causing cellular death (Babu et al. 1994). 

Most cells continually degrade and replace their membrane lipids. For each hydrolyzable bond in a glycerophospholipid, there is a specific hydrolytic enzyme in the lysosome (Fig. 10-15). Phospholipases of the A type remove one of the two fatty acids, producing a lysophospholipid. (These esterases do not attack the ether link of plasmalogens.) Lysophospholipases remove the remaining fatty acid. 

Certain classes of lipids are susceptible to degradation under specific conditions. For example, all ester-linked fatty acids in triacylglycerols, phospholipids, and sterol esters are released by mild acid or alkaline treatment, and somewhat harsher hydrolysis conditions release amide-bound fatty acids from sphingolipids. Enzymes that specifically hydrolyze certain lipids are also useful in the determination of lipid structure. Phospholipases A, C, and D (Fig. 10-15) each split particular bonds in phospholipids and yield products with characteristic solubilities and chromatographic behaviors. Phospholipase C, for example, releases a water-soluble phosphoryl alcohol (such as phosphocholine from phosphatidylcholine) and a chloroform-soluble diacylglycerol, each of which can be characterized separately to determine the structure of the intact phospholipid. The combination of specific hydrolysis with characterization of the products by thin-layer, gas-liquid, or high-performance liquid chromatography often allows determination of a lipid structure. 

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