Chemistry and biology of deoxy-myo-inositol phosphates: stereospecificity of substrate interactions within an archaeal and a bacterial IMPase

Six enantiomerically pure myo-inositol-monophosphates, including four deoxygenated analogues, have been synthesized by employing catalytic asymmetric phosphorylation methodology. These compounds were then evaluated as substrates for the direct interrogation of the stereospecificity of enzyme-substrate interactions with two inositol-monophosphatases (IMPases), one of which (from Archaeoglobus fulgidus) is characterized by an X-ray crystal structure with its substrate (d-I-1P) bound. The kinetic results lead to the finding that certain hydroxyl group contacts are actually destabilizing, while others have little effect. These new probes also allow a prediction of the active site binding mode of the substrate for the Escherichia coli IMPase for which no crystal structure exists.

The 6-OH group of D-inositol 1-phosphate serves as an H-bond donor in the catalytic hydrolysis of the phosphate ester by inositol monophosphatase

Inositol monophosphatase plays a pivotal role in the biosynthesis of secondary messengers and is believed to be a target for lithium therapy. It is established how a lithium ion works in inhibiting the enzyme but details of the mechanism for the direct magnesium ion activated hydrolysis of the substrate have been elusive. It is known that substrates require a minimal 1,2-diol phosphate structural motif, which in D-myo-inositol 1-phosphate relates to the fragment comprising the 1-phosphate ester and the 6-hydroxy group. Here it is shown that inhibitors that are D-myo-inositol 1-phosphate substrate analogues possessing 6-substituents larger than the 6-hydroxy group of the substrate, for example, the 6-O-methyl analogue, are able to bind to the enzyme in a congruous manner to the substrate. It is demonstrated, however, that such compounds show no substrate activity whatsoever. It is also shown that a 6-amino group is able to fulfil the role of the 6-hydroxy group of the substrate in conferring substrate activity and that a 6-methylamino group is similarly able to support catalysis. The results indicate that a 6-substituent capable of serving as a hydrogen-bond donor is required in the catalytic mechanism for hydrolysis. It has recently been shown that inositol is displaced from phosphorus with inversion of stereochemistry and we expect that the nucleophilic species is associated with Mg(2+)-1. It is proposed here that the role of the 6-hydroxy group of the substrate is to H-bond with a water molecule or hydroxide ion located on Mg(2+)-2. From this analysis, it appears that the water molecule bound to Mg(2+)-2 serves as a proton donor for the inositolate leaving group in a process that stabilises the alkoxide product and retards the back-reaction.

Inhibition of myo-inositol monophosphatase isoforms by aromatic phosphonates

alpha-Hydroxyphosphonates are moderately potent (Ki = 6-600 microM) inhibitors of the enzyme myo-inositol monophosphatase (McLeod et al., Med. Chem. Res. 1992, 2, 96). Hydroxy-[4-(5,6,7,8-tetrahydronaphtyl-1-oxy)phenyl]methyl phosphonate (3) was resynthesized and its inhibitory potency towards the recombinant bovine brain enzyme confirmed (Ki = 20 microM). Similar aromatic difluoro-, keto-, and ketodifluorophosphonates (5, 7, 9) were inactive. Compound 3 was 15-fold less active on the human as compared to the bovine enzyme. Molecular modeling suggested that the hydrophobic part of the inhibitor interacts with amino acid side chains that are located at the interface between the enzyme subunits in an area (amino acids 175-185) with low similarity between the two isozymes. Phe-183 in the human enzyme was replaced with leucine, the corresponding residue in the bovine isoform. The three isozymes (human wild-type, bovine wild-type and human F183L) had similar kinetic properties, except that the bovine enzyme was less effectively inhibited by high concentrations of the activator Mg2+. The F183L mutant enzyme had a twofold increased affinity for compound 3 as compared to the human wild-type form. We conclude that residue 183 contributes to the binding of aromatic hydroxyphosphonates to IMPase, but it is not the only determining factor for inhibitor specificity with respect to different isozymes.

Identification of rat liver glucose-3-phosphatase as an inositol monophosphatase inhibited by lithium

Glucose-3-phosphatase (Glc3Pase) from rat liver has been purified 780-fold with a 4% recovery. The substrate specificity of the purified enzyme agreed with that of inositol monophosphatase (EC 3.1.3.25). D-Glucose 3-phosphate (D-Glc(3)P; K(m) = 200 microM) was hydrolyzed with an efficiency similar to DL-myo-inositol 1-monophosphate (DL-Ins(1)P; K(m) = 80 microM), since the ratio V(max)/K(m) was similar for both substrates. Purification data, coelution of activities, thermal inactivation curves, optimal pH, bivalent cation requirements, inhibition by Li+, molecular weight, and isoelectric pH comparisons supported that the hydrolysis of D-Glc(3)P and DL-Ins(1)P was catalyzed by a unique phosphohydrolase identified as a hepatic form of the lithium-sensitive inositol monophosphatase. That the hydrolysis of D-Glc(3)P is a genuine feature of inositol monophosphatases was confirmed because the enzyme purified from bovine brain showed also Glc3Pase activity, and inspection of published 3D models of inositol monophosphatase complexes with D(L)-Ins(1)P or D(L)-Ins(4)P indicated that beta(alpha)-D-Glc(3)P in a pyranose conformation with all (but one) the hydroxy groups in equatorial orientation would fit in the active site as other good substrates do. The results of this work are suggestive of possible relationships between inositol and sugar 3-phosphate metabolism.

In vitro and in vivo inhibition of inositol monophosphatase by the bisphosphonate L-690,330

We have previously described the synthesis of bisphosphonate-containing inhibitors of inositol monophosphatase. In the present study, a more detailed examination of the in vitro and in vivo properties of one of these compounds, L-690,330, is described. L-690,330 is a competitive inhibitor of inositol monophosphatase with a Ki, depending on the source of IMPase, of between 0.2 and 2 microM. Although approximately 1,000-fold more potent in vitro than lithium, in muscarinic ml receptor-transfected Chinese hamster ovary cells prelabelled with [3H]inositol, L-690,330 only produced 40% of the accumulation of [3H]inositol monophosphates achieved by lithium at the same concentration (10 mM), suggesting that the ability of L-690,330 to cross the cell membrane is limited. Nevertheless, under conditions of cholinergic stimulation (100 mg/kg of pilocarpine s.c.), high doses of L-690,330 were able to increase brain inositol(l)phosphate levels in vivo to three- to fourfold control levels. This effect was dose dependent (ED50 = 0.3 mmol/kg s.c.) and was maximal after 1 h. In peripheral tissues, the effects of L-690,330 on inositol(l)phosphate levels mimicked those of lithium both qualitatively and quantitatively. However, in the brain, the effects of L-690,330 were much less than seen with lithium, consistent with the blood-brain barrier restricting access of the polar L-690,330 into the CNS, thereby further limiting entry of compound into cells in the brain. In the future, it may be possible to develop prodrugs of this compound, which circumvent many of the cell permeability problems inherent in bisphosphonate compounds.

Lithium and the phosphoinositide cycle: an example of uncompetitive inhibition and its pharmacological consequences

The ability of lithium to exert profound and selective psychopharmacological effects to ameliorate manic-depressive psychosis has been the focus of considerable research effort. There is increasing evidence that lithium exerts its therapeutic action by interfering with polyphosphoinositide metabolism in brain and prevention of inositol recycling by an uncompetitive inhibition of inositol monophosphatase. Stefan Nahorski, Ian Ragan and John Challiss discuss this unusual stimulus-dependent form of enzyme inhibition, emphasizing that the selectivity exhibited by lithium depends upon the degree of inositol lipid hydrolysis and polyphosphoinositide dephosphorylation.