Inositol-cleaving enzyme

Inositol-cleaving Enzyme from Rat Kidney. Methods in Enzymol. 5, 329 (1962). Reddy, C. C., P. A. Pierzchala, and G. A. Hamilton Effects of Various Metabolites, Complexing Agents and Metal Ions on Inositol Oxygenase. Fed. Proc., Fed. Amer. Soc. Exp. Biol. 37, 1720 (1978). 

Phosphohpase D cannot cleave phosphatidyl inositol. The enzyme is present in cereals, such as rye and wheat, and in legumes. It was isolated and purified from peanuts. 

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... 

A selection of other tumor suppressor genes is summarized in Table 14.2. Interestingly, an enzyme of phosphatidyl-inositol metabolism has been also identified as a tumor suppressor. The PTEN tumor suppressor gene codes for a phospholipid phosphatase which specifically cleaves a phosphate from the second messenger phosphatidyl-inosi-tol-3,4,5-trisphosphate (PtdInsPj, see 6.6.2). and thus inactivates the messenger (review Maehama and Dixon, 1999). ... 

Figure 1. Inositol synthesis and catabolism. Inositol is synthesized from glucose-6-phosphate (Glucose 6-P) by the action of L-wyo-inositol 1-phosphate synthase (MIPS) and wyo-inositol monophosphatase (IMP). IMPase is also required for the last step of inositol (1,4,5)P3 second messenger breakdown. The first step in inositol catabolism utilizes the wyo-inositol oxygenase (MIOX) enzyme, which cleaves the inositol ring to form D-glucuronic acid.

As discussed in Chapter 13, hormonal stimulation of some G protein-coupled receptors leads to activation of the (3 isoform of phospholipase C (PLCp). This membrane-associated enzyme then cleaves phosphatidylinositol 4,5-bisphosphate (PIP2) to generate two important second messengers, 1,2-diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). Signaling via the IP3/DAG pathway leads to an increase in cytosolic Ca " and to activation of protein kinase C (see Figure 13-29). 

Two classes of enzymes that degrade myoinositol hexakisphosphate are recognized 3-phytase (EC 3.1.3.8) initially cleaves the phosphate at the 3-position on the myoinositol ring, leaving myo-inositol 1,2,4,5,6 pentakisphosphate (this phytase belongs to the family of histidine acid phosphatases Mitchell et al, 1997), whereas 6-phytase (EC 3.1.3.26) preferentially initiates dephosphorylation at the 6-position, leaving myoinositol 1,2,3,4,5 pentakisphosphate. 

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