Polyphosphoinositides degradation

Verghese, M. W., Smith, C. D., and Snyderman, R. (1986). Role of guanine nucleotide regulatory protein in polyphosphoinositide degradation and activation of phagocytic leukocytes by chemoattractants. J. Cell Biochem. 32, 59-69. 

In rabbit iris sphincter smooth muscle microsomal fraction, there are phosphomonoesterases that degrade IP3 to IP2, IP2 to IP, and IP to free myo-inositol and Pj (Abdel-Latif, 1986). The IP3 phosphatase has been shown to specifically remove the 5-phosphate from IP3 and from cyclic IP3 to produce IP2 and cyclic IP2, respectively. The polyphosphoinositide phosphatase has also been reported to dephosphorylate inositol tetrakisphophate (IP4) to IP3. These enzyme activities are both cytosolic and membranous, dependent on Mg2+, and not inhibited by Li+. The IP3 5-phospha-tase was studied in the microsomal fraction of bovine iris sphincter muscle (Wang et al., 1994). It hydrolyzed IP3 to I(1,4)P2 with an apparent of 28 (xM Mg2+ was required for its activity, Ca + (> 0.5 (xM) was inhibitory, and Li+ or phosphorylation of the microsomal fraction with cAMP-dependent protein kinase or protein kinase C (PKC) had no effect on the activity of the enzyme. 

Polyphosphoinositides are readily broken down in tissue preparations. Thompson and Dawson (1964 a) showed that in extracts of brain tissue polyphosphoinositides may be degraded by one of two pathways. For example, triphosphoinositide may be hydrolysed by a specific triphosphoinositide phosphodiesterase to yield diglyceride and inositol triphosphate ... 

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