Kinetic and inhibitor profiles of soluble and particulate inositol 1,4-5-trisphosphate 5-phosphatase from GH3 and IMR-32 cells

beta-Amyloid(25-35) inhibits the activity of inositol(1,4,5)-trisphosphate-5-phosphatase

beta-amyloid (A beta) peptides are known to disrupt calcium homeostasis in cells. In the present study, the effects of A beta(25-35) upon the activity of soluble Ins(1,4,5)P3-5-phosphatase have been investigated. A beta(25-35) inhibited, and dithiothreitol (DTT) increased the activity of soluble rat cerebellar Ins(1,4,5)P3-5-phosphatase. The change in activity was not accompanied by an obvious change in the sensitivity of the Ins(1,4,5)P3-5-phosphatase to inhibition by glucose-6-phosphate or phytic acid. A beta(35-25) also inhibited soluble Ins(1,4,5)P3-5-phosphatase activity, but at a lower potency than A beta(25-35). It is concluded that A beta(25-35) affects the metabolism of Ins(1,4,5)P3 although the potency is not sufficiently high to contribute to any significant extent to the effects of this peptide upon calcium homeostasis.

Inhibition of inositol 1,4,5-trisphosphate 5-phosphatase by micromolar concentrations of disulfiram and its analogues

Following a preincubation period of 10 min, disulfiram and its analogues FLA 46, FLA 63, FLA 99, EWP 815 and EWP 840 inhibited the breakdown of 10 microM [3H]Ins(1,4,5)P3 by Ins(1,4,5)P3 5-phosphatase from GH3 cells, with IC50 values (in microM), for soluble/particulate enzymes respectively, of: disulfiram, 24/24; FLA 46, 23/30; FLA 63, 24/6; FLA 99, 50/48; EWP 815, 8/6; EWP 840, 11/8. The inhibition produced by FLA 99 was time-dependent in nature, although inhibition was found in the absence of a preincubation period. EWP 815 and EWP 840 were more potent inhibitors of Ins(1,4)P2 phosphatase than of Ins(1,4,5)P3 5-phosphatase. Thyrotropin-releasing hormone (TRH; 3/100 microM)-stimulated inositol phospholipid breakdown in prelabelled GH3 cells was inhibited by disulfiram (IC50 values 63/52 microM respectively), FLA 46 (89/110 microM), EWP 815 (83/71 microM) and EWP 840 (220/200 microM), without affecting basal breakdown rates. FLA 99 did not inhibit either basal or TRH-stimulated activity at any of the concentrations tested (30, 100 and 300 microM). [3H]Ins(1,4,5)P3 binding to its cerebellar receptor was not inhibited by any of the compounds over a concentration range of 3-300 microM, although an increased level of binding was seen at high concentrations. FLA 99 and EWP 840 increased the basal intracellular Ca2+ concentration in GH3 cells, but with no corresponding effect on the Ca2+ response to TRH stimulation. These compounds did not increase the cellular permeability to Trypan Blue, but did affect cell proliferation. It is concluded that disulfiram and related compounds produce dramatic effects on Ins(1,4,5)P3 metabolism in GH3 cells.

Time-dependent inhibition of inositol-1,4,5-trisphosphate-5-phosphatase by calmidazolium chloride in rat GH3 cells

The calmodulin inhibitor calmidazolium chloride inhibited the activity of soluble and particulate Ins(1,4,5)P3-5-phosphatase from GH3 cells, with an IC50 value of approximately 100 microM following a 10-min preincubation with the enzyme. The inhibition was time-dependent and could not be reversed by washing of the particulate fraction. It is concluded that although the inhibitory effect of calmidazolium chloride cannot be related per se to inhibition of calmodulin function, effects of this compound unrelated to actions upon calmodulin function may be found when concentrations that are only moderately supramaximal are used.