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Kinetic characterization of enzyme forms involved in metal ion activation and inhibition of myo-inositol monophosphatase


, : Kinetic characterization of enzyme forms involved in metal ion activation and inhibition of myo-inositol monophosphatase. Biochemical Journal 307: 585-593

Activation and inhibition of recombinant bovine myo-inositol monophosphatase by metal ions was studied with two substrates, D,L-inositol 1-phosphate and 4-nitrophenyl phosphate. Mg-2+ and Co-2+ are essential activators of both reactions. At high concentrations, they inhibit hydrolysis of inositol 1-phosphate, but not 4-nitrophenyl phosphate. Mg-2+ is highly selective for inositol 1-phosphate (k-cat = 26 s-1) compared with the aromatic substrate (k-cat = 1 s-1), and follows sigmoid activation kinetics in both cases. Co-2+ catalyses the two reactions at similar rates (k-cat = 4 s-1), but shows sigmoid activation only with the natural substrate. Li+ and Ca-2+ are uncompetitive inhibitors with respect to inositol 1-phosphate, but non-competitive with respect to 4-nitrophenyl phosphate. Both metal ions are competitive inhibitors with respect to Mg-2+ with 4-nitrophenyl phosphate as the substrate. With inositol 1-phosphate, Ca-2+ is competitive and Li+ non-competitive with respect to Mg-2+. Multiple inhibition studies indicate that L-i+ and P-i can bind simultaneously, whereas no such complex was detected with Ca-2+ and P-i. Several sugar phosphates were also characterized as substrates of myo-inositol monophosphatase. D-Ribose 5-phosphate is slowly hydrolysed (k-cat = 3 s-1), but inhibition by Li+ is very efficient (K-i = 0.15 mM), non-competitive with respect to the substrate and competitive with respect to Mg-2+. Depending on the nature of the substrate, Li+ inhibits by preferential binding to free enzyme (E), the enzyme-substrate (E cntdot S) or the enzyme-phosphate (E cntdot P-i) complex. Ca-2+, on the other hand, inhibits by binding to E and E cntdot S, in competition with Mg-2+. The results are discussed in terms of a catalytic mechanism involving two metal ions.

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PMID: 7733900


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