Role Of Inositol Monophosphatase In Bipolar Disorder Biology Essay

In encephalon cells of patients enduring from bipolar upset there is a chemical instability due to increase in concentration of inositol. Lithium is an effectual temper stabilizer for bipolar upset patients and its curative consequence may affect suppression of inositol monophosphatase activity in the PI signaling pathway. Lithium besides affects other enzymes and facets in the PI signaling pathway so it is non precisely clear whether transition of PI responses can be attributed entirely to the suppression of IMPase. Lithium therapy has a batch of side effects and a narrow curative window, so there is demand to plan an organic IMPase inhibitor as all the current IMPase inhibitors are to polar to be used as drug molecules. Drugs that mimic Li with its regard to curative mechanism of action might stand for a fresh attack to the intervention of bipolar upset. Lithium is still being used despite its many side effects and narrow curative window.


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“ Bipolar upset is a common terrible, chronic and life threatening unwellness where patients alternate between episodes of depression and passion ”

Why bipolar upset is a job and why the current drugs do n’t work. Bipolar upset affects around 340million people worldwide. It is a terrible job because it has no remedy and the mean continuance of episodes of passion is around 2-3months. The current intervention is lithium therapy. As a consequence of the side effects profile of Li, an appreciable figure of bipolar patients can non digest Li therapy.

Signs and symptoms ~ non everyone ‘s symptoms are the same and no simple physiological trial to corroborate

Diagnosis~ how it is assessed~ interview and analyze the patient, take history from an source, measure their hazard to themselves or to others

The likely causes are ( multifactorial ) complex~ familial factors ( biological ) environmental ( physical and psychosocial ) and hormonal abnormalcies ( chemical instability in the encephalon )

Section 1

The function of inositol Monophosphatase

IMPase is a cardinal enzyme in a encephalon secondary courier system, it provides free myo-inositol for the biogenesis of the secondary courier precursor Phosphatidylinositol-4, 5-bisphosphate. Inositol monophosphatase besides hydrolyses inositol monophosphate, inositol 3-phosphate and inositol 4-phosphate to inorganic phosphate and inositol, which is the precursor for all inositol-based signaling go-betweens. The over activity of this cellular response mechanism is linked to the violent temper swings associated bipolar patients. The Suppression of IMPase activity by an inhibitor such as Li reduces the degrees of myo-inositol in the encephalon and therefore slows down the resynthesis of 2nd courier precursors.

Figure 1: The hydrolysis of Myo-inositol monophosphate catalysed by inositol monophosphatase

Scheme 1: The hydrolysis of myo-inositol monophosphate by IMPase

The cistron that codes for Inositol monophosphosphatase 2 ( IMPA2 ) is located in 18p11.2 ; this is a bipolar-disorder susceptible venue. Polymorphism of individual base in the booster and coding part of IMPA2 has been strongly linked to bipolar-disorder.

( PI ) cell signalling and bipolar upset

In response to an external stimulation, Phosphatidylinositol-4, 5- bisphosphate is hydrolysed to the secondary couriers inositol-1, 4, 5-trisphosphate and 1, 2- diacylgycerol. Once Inositol-1, 4, 5-trisphosphate has been released it is dephosphorylated by enzymes to finally give myo-inositol. IMPase catalyses the concluding measure in this sequence of dephosphorylation ( figure2 ) . There is an unnatural addition in the myo-inositol degrees in encephalons of bipolar patients and these additions of the resynthesis of the secondary courier precursors. The over activity of this cellular response mechanism is thought to do the violent temper swings associated bipolar patients.


Inositol ( 1,3,4,5 ) P4

Inositol ( 1, 4,5 ) P3

Phosphatidyl-inositol ( 4, 5 ) P2

Inositol ( 1,3,4 ) P3

Inositol ( 3,4 ) P2

Inositol ( 1,3 ) P2

Inositol ( 1,4 ) P2



Inositol ( 1 ) P inositol ( 3 ) P Inositol ( 4 ) Phosphorus

( 1 ) Phosphorus




Figure2: The Phosphatidylinositol tract demoing the inositol rhythm

Evidence of the function of inositol Monophosphatase in bipolar upset and in antibipolar intervention ( The inositol depletion hypothesis )

There is an premise inositol Monophosphatase is involved in bipolar upset, from the hypothesis that Li treats bipolar upset by cut downing the responses of the PI signaling system and PI neurotransmitters that are overactive during depression or passion. But there is no direct grounds to demo that a dysfunctional PI signalling is associated with the pathophysiology of bipolar upset

Lithium reduces the degree of inositol in the encephalon by its suppression of the enzyme inositol monophosphatase. Inositol required for the resynthesis of PI is derived from the dislocation of inositol monophosphate by inositol monophosphatase. The enzyme inositol monophosphatase was measured in human ruddy blood cells of psychiatrically healthy control topics, Lithium-free bipolar patients, and Lithium-treated bipolar patients and was found to be reduced by about 80 % in Li-treated bipolar patients, this findings support the construct that chronic Lithium at curative concentrations inhibits inositol monophosphatase.

1.4 The crystal construction of human Myo-Inositol Monophosphatase ( substrate binding )

Inositol monophosphatase is a dimeric protein incorporating 277 aminic acids in each monomer unit ( figure 3 ) . The enzyme exists as a dimmer of indistinguishable sub-units each folded into a five superimposed sandwich of three braces of alpha spirals and two beta sheets in a I±I?I±I?I± agreement.

This crystal construction with Gd sulfate edge ( Gd3+ ) , high spots Mg2+ ( 1 ) metal adhering site as Glu-70, Asp-90, Ile -92, Thr-95, the sulfate binds to metal ion. This construction suggested that there was one metal adhering site per fractional monetary unit this was because the crystal construction was obtained in the presence of high concentrations Li which was bound to the 2nd metal adhering site.

Figure3: The crystal construction of human inositol monophosphatase determined by utilizing x-ray crystallography. The sulfate and Gd3+ are bound at indistinguishable sites on each of the fractional monetary units and take the place of the active sites.

Gd3+ and sulfate are competitory inhibitors for Mg2+ and phosphate. Beginning: hypertext transfer protocol: // bioNum=1

To find the construction of the active enzyme complex the Gd3+ ion in the crystal construction was replaced by Mg2+ , and the sulfate group was replaced with the phosphate group of the substrate. It was found that the place of Mg2+ ( 1 ) in the substrate composite corresponded to the place of Gd3+ in the x-ray construction for inositol monophosphatase substrate composite. The active enzyme composite showed that Mg2+ ( 1 ) co-ordinates to the hydroxy group of Thr-95, the side concatenation carboxylate groups of Glu-70 and Asp-90, to the carbonyl O-atom of IIe-92 every bit good to the two non-bridging O-atoms of the phosphate group as shown in figure 4. The H2O molecule is unable to co-ordinate with the phosphate group of the substrate.Mg2+ 1gets to the active site before any of the other species, and it remains there throughout the whole catalytic procedure, even at high saturating concentration of the substrate

Figure 4: The coordination domain of Mg2+ 1 in the optimised construction of Mg2+ enzyme substrate composite


The 2nd metal active site for Mg2+ is formed in the enzyme substrate composite, with ligands from the substrate and enzyme. Mg2+ is co-ordinates to the substrate through the bridging ester O-atom and one of the tantamount O-atoms of the phosphate group, it coordinates to the enzyme through the carboxylate groups of the three aspartate groups, Asp-90, 93 and 220 ( figure 5 ) .

Figure 5: The coordination domain of Mg2+ 2 in the Optimised construction of Mg2+ enzyme substrate composite.


The cardinal function of Mg2+ 2 ion is to move as a Lewis acid and deprotonate the H2O molecule ( nucleophile )

Table 1: The key amino acids involved in substrate and metal binding


Amino acids

Substrate binding







Mg2+Site 1





Mg2+Site 2




Nucleophilic H2O activation



( beginning: Bipolar medicines mechanisms of action )

Section 2: Chemistry of Li ( Li and IMPase metal adhering ) ~ Li and enzymes/ proteins~ cardinal active site interactions~ Mechanisms.

2.1 How does lithium work ~ what Li affects in the encephalon

Li and the Phosphatidylinositol ( PI ) signalling tract

IMPase the chief mark for Li


Bipolar upset

PI signaling

The anti-manic consequence of Li was foremost discovered by chance by an Australian psychologist John Cade in 1949. Inhibition of Inositol Monophosphatase a cardinal enzyme ( critical for the regeneration of secondary courier ) in the PI signalling pathway produce the effects on the PI signalling that mimic those effects produced by Li. Pharmacological intercession aimed at suppressing inositol monophosphatase, by Li therapy, leads to a depletion of free available inositol.

2.2: Interactions between inositol monophosphate, Li and its substrate

The substrate inositol monophosphate binds to inositol monophosphatase with mg2+ in metal binding site 1 ( Mg2+ remains bound to the enzyme throughout the procedure ) . A 2nd Mg2+ binds ( after substrate binding ) in metal binding site 2 and interacts with the three aspartate residues ( Asp 90, 93 and 220 ) . The binding of the 2nd Mg2+ activates the inositol ester O and makes it more susceptible to nucleophilic onslaught by H2O molecule activated by site 1 mg2+ , Glu70 and Thr95. The nucleophilic onslaught consequences in the hydrolysis of the phosphate bond, let go ofing inositol and go forthing the inorganic phosphate with both Mg2+ ions. In normal conditions the Mg2+ in site 2 and the inorganic phosphate unbinds to renew the enzyme with Mg2+ edge in site 1, for the hydrolysis of the substrate in the following unit of ammunition. But in the presence of Li ( scheme2 ) , when the Mg2+ in site 2 unbinds, the Li occupies this site. This prevents the inorganic phosphate from unbinding ; this leaves inositol monophosphatase in an inactive province

Scheme 2: sum-up of the mechanism of substrate hydrolysis and suppression by Li of inositol monophosphatase

E = enzyme

InsP = inositol monophosphate

Li= Li

Pi = inorganic phosphate

Mg ( 1 ) and Mg ( 2 ) binding at metal site1 and 2 severally

InsP Mg

E.Mg ( 1 ) . E.Mg ( 1 ) .InsP


E.Mg ( 1 ) .InsP.Mg ( 2 )

E.Mg ( 1 ) .Pi E.Mg ( 1 ) .Pi.Mg ( 2 )

Li Mg Ins

E.Mg ( 1 ) .Pi.Li

Kinetic surveies on IMPase

At low Mg2+ concentrations each monomer is required to adhere with one Mg2+ ion for full activity, and the binding of the 2nd Mg2+ after substrate binding is facilitated by the binding of the first ion. The enzyme is inhibited at high concentrations of Mg2+ .Lithium non-competitively inhibits IMPase with regard to its substrate at low concentration or high concentrations.

Structural demands for substrate activity

The inositol pealing binds to the active site cleft through its hydroxy groups, 2-OH, and 4-OH are involved in adhering to the enzymes. 6-OH is involved in contact action ( remotion of the 6hydroxy group stops catalytic activity, it coordinates to the H2O molecule ( maps as the nucleophile ) to finish the shell of Mg2+ 2 to give the octahedral geometry. Neither the 3-OH nor 5-OH has an consequence on contact action or adhering.2-OH signifiers H-bonding with the side concatenation of Asp-93, and to Ala-196. 4-OH signifiers H-bonding with the carboxylate group of Glu-213.

The active site nucleophile is a H2O molecule coordinated to the 2nd Mg2+ ion ~ Mg2+ activates the nucleophile. The nucleophile ( Mg2+ 2 edge H2O molecule ) attacks the phosphoric from the opposite side of the O-atom which is H bonded to the NH of Gly-94 and Thr-95. In the merchandise composite formed the side concatenation of Thr-95 and Mg2+ 1 interact with the imposter rotated phosphate O-atom ( keeping of stereochemistry ) . This agreed with the x-ray crystal ( inactive ) construction of the di-Mn2+ phosphate merchandise composite.

In-line nucleopilic onslaught ( by Merck, Sharp and Dohme ) ~ ensuing in inversion of constellation

Harmonizing to the writers ~Mg2+ 1 bound H2O molecule is in H-bonding with the 3-OH group of Thr-95 and is activated by Glu-70. ( Implies a lessening in nucleophility )

The comparing of in-line and pseudo rotary motion mechanisms.

2.3 Problems associated with lithium~ complications of Li intervention. The Pharmacological facets of Li: Dose, soaking up, distribution, elimination and the effects of Li.

Lithium is given orally as the carbonate salt ( lithium carbonate ) or as a liquid ( lithium citrate ) . It is quickly absorbed from the intestine within 8hours of disposal. Li+ does non adhere to serum proteins and it is equally distributed in entire organic structure H2O both intra and extracelularly. It can non be pumped out of cells every bit expeditiously as Na but it can be actively transported across cell membranes. Li+ uptake into tissues is non unvarying ; it is quickly taken up by the kidney and more easy into the liver, musculus and bone. The distribution of Li in the encephalon is even slower and delayed by 24hours compared with plasma. Lithium is non equally distributed besides between the serum and the CSF, the concentration of Li in the cerebrospinal fluid and serum is a 1:3 to 1:4 ratios. This is due to its conveyance out of the cerebrospinal fluid by the encephalon capillary endothelium.

Elimination of Li is via the kidney ; about half of the unwritten dosage is excreted within 12hours. The balance is taken up by cells and excreted over 1-2 hebdomads. This means that with regular dose Li accumulates easy over 2weeks before a steady province is reached. The narrow curative window ( 0.5-1.5mM ) and long continuance of action means that the dosage and plasma concentration must be monitored. The chief toxic effects that occur during intervention include sickness, purging, shudder of the custodies, polydipsia and polyuria. Signs of Li toxicity include sleepiness and confusion, plasma concentrations above 2-3mM, dangerous ictuss and coma may happen and decease if the with plasma concentration reaches 3-5mM.

2.4 Alternatives inhibitory of inositol Monophosphatase

Substrate based inhibitors ~

L-690,330 competitory inhibitor of IMPase synthesised on the footing of the substrate inositol monophosphatase, really powerful, at really high concentrations was able to bring forth an accretion of inositol monophosphate, but L-690, 330 could non traverse the cell membrane in tissue civilization so the prodrug L-690,488 was produced by esterification of the bisphosphonate group, to cut down the high charge.

Gani group phosphate inhibitors ~ substrate like inhibitors possessing 6-substituents other than the 6-hydroxy group of the substrate, where able to adhere to the enzyme. With the exclusion of 6-amino phosphate and 6-methlamino phosphate ( km values of 300AµM and 140AµM severally ) this had hydrogen bond giver at C 6. This helped to set up that primary and secondary amino groups at C 6 where able to interact with Mg2+ ion edge to H2O molecule. The staying phosphate parallels showed no substrate activity.

Phosphate inhibitors synthesised by the Merck group~ Series of inhibitors made by the omission of each hydroxyl group in the natural substrate, some turned out to be extremely powerful inhibitors of IMPase.

Terpenoid inhibitors ~ Two structurally similar inhibitors where Isolated from Memnonniella ( Ki value of about 500 AµM ) and Stachybotris species ( Ki value 450AµM ) . They were both non-competitive inhibitors with regard to inositol 3 phosphates and Mg2+ with a different suppression profile to Li.

Tropolone Inhibitors ~ Puberulonic acid was a competitory inhibitor of IMPase isolated from several Penicillium strains, with an IC50 value of 10AµM. Although they are less polar than the substrate based inhibitors their bioavailability is excessively low to be used as drugs.

Section 3 ; Central consequence of Li grounds for other enzymes involved in bipolar upset

3.1 The alternate marks of lithium~ .

3.2 Li ‘s consequence on cyclic AMP

Lithium inhibits endocrine induced camp production and blocks cellular responses, such as the response of thyroid to thyroid stimulation endocrine and the nephritic tubular cells to antidiuretic endocrine although it does non hold a marked consequence in the encephalon.

3.3 Lithium ‘s consequence on protein kinase C ( PKC )

Protein kinase C might play an of import function in some of the cognitive characteristics of passion. PKC activation consequences in frenzied like behavior while suppression of PKC consequences in behavior that might be antimanic in knock-out mice. Lithium consequences in increased concentration of diacylgycerol and this reduces the activity of protein kinase C and

3.4 Lithium ‘s consequence on animal starch synthase kinase 3 ( GK3 )

Lithium inhibits animal starch synthase kinase 3, which phosphorylates the cardinal enzymes involved in the tract taking to amyloid formation and programmed cell death. GSK3 suppression resulted in behavioral alterations ( in knock -out mice ) that are relevant to both passion and depression. From the behavioral informations sing GSK3 in smasher mice it suggested that Li ‘s curative consequence comes from its activity as a GSK3 inhibitor.

Section 4 ; Alternative drugs in the intervention of bipolar upset and, Future theoretical accounts of new drugs

4.1 temper bracing medicines: Valproic acid ( VPA and inositol depletion ) and Carbamazepine.

Valproic acid and Carbamazepine are used to handle frenzied episodes in bipolar patients that are unresponsive to lithium. These drugs are deriving favor for the intervention of bipolar patients because of their less side effects and safety profile. These drugs are antiepileptics and are normally used to handle epilepsy, but are now an option to Li. Valproate produces altered forms of look in of protein kinase C, thereby changing cellular response and cistron look.

4.2 Why Li is better in handling bipolar upset.

Lithium is more effectual at forestalling episodes of passion in bipolar upset than handling it. In Contrast to lithium Valproic acid and Carbamazepine are thought to be better in forestalling the return of depressive episodes in bipolar patients.

4.3 Why the new drugs fail~ non polar, tilt cross the blood encephalon barrier,

Presently all the IMPase inhibitors which are powerful plenty to be used as drug molecules are to polar to traverse cell membranes and be used as drugs

L-690,330 ~ could non perforate the cell membrane because of its extremely charged bisphosphonate, alternatively the tetrapivaloymethyl ester of prodrug of this compound L-690,488 was synthesised. L-690,488 was more permeable than L690, 333 and it served as a prodrug let go ofing the parent compound L-690,333 when in the cell, that will so be able to suppress IMPase. But this besides failed in vivo as a consequence of its lipohilicity, when injected into animate beings neither it nor the parent compound L-690,333 could be detected in the plasma or encephalon. This suggested that the compound remained at the site of injection.

Puberulonic acid ~ the anhydride was supposed to hydrolyze to give biscarboxylic acid and biscarboxylic acid was supposed to chelate Mg2+ ion in the active site of IMPase, but the biscarboxylic acid was shown to the inactive

4.4 Future theoretical accounts development of new compounds with the clinical profile of Li with less side effects and a narrow curative window. IMPase inhibitors ( future mark ) mimic the effects of Li on the PI rhythm

The substrate based inhibitors of IMPase were non suited for usage in in-vivo


Design of bioavailable drug molecules for IMPase, ( construction based drug design )

More probe into the IMPase mechanism ( examining the function of the catalytic H2O molecule )

Design of a pharmacophore ~ FOR PHOSPHATE BINDING INHIBITORS


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