Purification of an inositol (1,3,4,5)-tetrakisphosphate 3-phosphatase activity from rat liver and the evaluation of its substrate specificity

Abstract

Hepatic inositol (1,3,4,5)-tetrakisphosphate 3-phosphatase activity was detected in a 100,000 x g soluble fraction and a detergent-solubilized particulate fraction. Activity in both fractions increased up to 40-fold after anion-exchange chromatography due to removal of endogenous inhibitors (Hodgson, M.E., and Shears, S.B. (1990) Biochem. J. 267, 831-834); at this stage the detergent-solubilized particulate activity comprised over 90% of total activity. The particulate phosphatase was further purified by affinity chromatography using heparin-agarose and red-agarose. The latter column resolved two peaks of enzyme activity (designated 1 and 2 by their order of elution from the column). Their proportions varied between experiments, but peak 2 generally predominated and so this was further purified by hydroxylapatite chromatography. The final preparation was typically 38,000-fold purified with a 7% yield. The apparent molecular mass of this enzyme was 66 kDa, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration. The enzyme had little or no affinity for the following: inositol (1,3,4,6)-tetrakisphosphate, inositol (1,3,4)-trisphosphate, inositol (1,3)-bisphosphate, inositol (3,4)-bisphosphate, and para-nitrophenylphosphate. At pH 7.4 the Km for inositol (1,3,4,5)-tetrakisphosphate was 130 nM and the Vmax was 4250 nmol/mg protein/min. The purified enzyme also dephosphorylated inositol (1,3,4,5,6)-pentakisphosphate to inositol (1,4,5,6)-tetrakisphosphate (Km = 40 nM, Vmax = 211 nmol/mg protein/min), and inositol hexakisphosphate to at least five isomers of inositol pentakisphosphate (Km = 0.3 nM, Vmax = 12 nmol/mg protein/min). The latter affinity is the highest yet defined for an enzyme involved in inositol phosphate metabolism. Determinations of IC50 values, and Dixon plots, revealed that with the (1,3,4,5)-tetrakisphosphate as substrate, the pentakis- and hexakisphosphates were potent competitive inhibitors; the Ki values (25 and 0.5 nM, respectively) were similar to their substrate Km values. The kinetic properties of this enzyme, as well as estimates of the cellular levels of its potential substrates, indicate that inositol pentakisphosphate and inositol hexakisphosphate are likely to be the preferred substrates in vivo.