Inositol 1,3,4,5,6-pentakisphosphate and inositol hexakisphosphate inhibit inositol-1,3,4,5-tetrakisphosphate 3-phosphatase in rat parotid glands.
In assays containing a physiological concentration of inositol 1,3,4,5-tetrakisphosphate (1 microM), this isomer was attacked by both 3- and 5-phosphatases present in rat parotid homogenates and 100,000 X g supernatant and particulate fractions. As the concentration of cytosolic protein in the assay was decreased, the specific activity of the soluble 3-phosphatase increased significantly. In contrast, the specific activity of particulate 3-phosphatase was independent of protein concentration. At the lowest protein concentrations tested, the sum of soluble and particulate 3-phosphatase specific activities was 2.5-fold greater than that of the parent homogenate. These observations indicate that parotid cytosol contains a hitherto undescribed endogenous mechanism for inhibiting 3-phosphatase. The effects upon 3- and 5-phosphatase of a number of inositol polyphosphates were studied. Both activities were inhibited by inositol 1,4,5-trisphosphate and inositol 1,3,4-trisphosphate (IC50 approximately 50 microM). Inositol 3,4,5,6-tetrakisphosphate was a more potent inhibitor of 3-phosphatase (IC50 about 10 microM) and did not affect 5-phosphatase. Inositol 1,3,4,5,6-pentakisphosphate and inositol hexakisphosphate were very potent inhibitors of 3-phosphatase (IC50 values of 1 and 0.5 microM, respectively); these polyphosphates did not affect 5-phosphatase activity at concentrations of up to 10 microM. Inositol 1,3,4,5,6-pentakisphosphate was a competitive inhibitor of the 3-phosphatase, whereas inositol hexakisphosphate was a mixed inhibitor. These data lead to the proposal that the inositol 1,3,4,5-tetrakisphosphate 3-phosphatase is unlikely to be an important enzyme activity in vivo.
Hughes
P J
PJ
Inositol Lipid Section, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, North Carolina 27709.
Shears
S B
SB
D000818:Animals; D001667:Binding, Competitive; D003600:Cytosol / Q000201:enzymology; D015544:Inositol 1,4,5-Trisphosphate / Q000302:isolation & purification / Q000378:metabolism; D007295:Inositol Phosphates / Q000494:pharmacology*; D000072181:Inositol Polyphosphate 5-Phosphatases; D007700:Kinetics; D010306:Parotid Gland / Q000201:enzymology*; D010744:Phosphoric Monoester Hydrolases / Q000037:antagonists & inhibitors*; D010833:Phytic Acid / Q000494:pharmacology*; D051381:Rats
D007295:Inositol Phosphates; C013571:inositol pentaphosphate; D010833:Phytic Acid; D015544:Inositol 1,4,5-Trisphosphate; D010744:Phosphoric Monoester Hydrolases; D000072181:Inositol Polyphosphate 5-Phosphatases