Lathakumari Parthasarathy

Biochemical and molecular properties of lithium-sensitive myo-inositol monophosphatase

Myo-inositol monophosphatase is a pivotal enzyme of the inositol second messenger system which is specifically inhibited by therapeutic levels of lithium salts, implicating inhibition of this enzyme as a potential site of its action in bipolar disease. This enzyme has a native molecular weight of 59,000, and has traditionally been found in the cytosolic fraction, although a membrane-bound form has also been identified. Possessing two identical subunits, this enzyme hydrolyzes those monophosphates which are equatorially located within the inositol ring, and several nucleoside monophosphates phosphorylated at the 2-position. Each subunit of the native enzyme contains an active site with unusually large caverns as revealed by crystallographic studies, which may explain the accommodation of these structurally unrelated substrates. We have suggested that the uncompetitive inhibition of this phosphatase by lithium ions may prevent the formation of an enzyme-bound non-isomeric (meso) intermediate, Mg(2+)-inositol 1,3 or 4,6 cyclic monophosphate when this enzyme hydrolyzes its respective isomeric substrates.

Role of Inositol in the Treatment of Psychiatric Disorders

SummaryMyo-inositol is a ubiquitous carbohydrate that is present in large amounts in brain tissue and is involved in neuronal signalling and osmoregulation. This sugar is an essential component of the inositol signalling system, which is a post-receptor second messenger signalling system found in many cells. Myo-inositol is the precursor of membrane inositol phospholipids, which are critically linked to a number of CNS receptor signalling systems, including muscarinic, serotonergic, adrenergic, metabotropic and histaminergic systems, and those linked to cholecystokinin, tachykinins, neurotensin, platelet activating factor and other transmitters.Upon stimulation of these receptors, a signal is transmitted through a guanosine triphosphate (GTP)-binding protein (Gq), which then activates the enzyme phospholipase C. This results in the release of a second messenger, inositol 1,4,5-trisphosphate (InsP3), from membrane inositol phospholipids. InsP3 then causes the release of free intracellular calcium into the cytosol, activating a number of enzymes or receptors. Myo-inositol in the brain is derived from 3 sources: (i) receptor stimulation (a salvage pathway); (ii) de novo synthesis from glucose; and (iii) uptake of dietary myo-inositol through plasma membrane myo-inositol transporters. Most myo-inositol is probably derived from the first 2 sources, which are controlled through the lithium-sensitive enzyme myo-inositol monophosphatase (IMPase). This enzyme acts upon myo-inositol monophosphates, hydrolysing them to release free myo-inositol. Recent biochemical, molecular and crystallographic studies have demonstrated that the overall metabolism of brain inositol is closely modulated by this enzyme. Lithium salts, which are commonly used in various psychiatric conditions, inhibit this enzyme, and this action has been implicated as a therapeutic mechanism of action of lithium.A change in the availability of CNS inositol may lead to altered brain cell signalling pathways and, eventually, to the development of a neuropsychiatric disorder. Recent evidence indicates that myo-inositol has psychoactive effects, with initial studies demonstrating effectiveness in the treatment of depression, panic disorder and obsessive-compulsive disorder. At present, the exact mechanism of these clinical effects is uncertain. The development of various inositol system-based drugs may lead to future psychoactive drugs designed to modulate a second messenger cascade of events rather than a receptor system, and will lead to further understanding of CNS disease from a post-receptor second messenger perspective.

Promising Psychotherapeutic Effects of the Natural Sugar: Myo-Inositol.

Myo-inositol is a common six-carbon sugar with unique biochemical and psychotherapeutic properties. It is involved in neuronal signaling and osmoregulation, and has been shown to be therapeutic in initial studies of depression, panic disorder, and obsessive-compulsive disorder. The inositol signaling system is a post-receptor second messenger system found in many cells, and is similar to the cAMP system. Myo-inositol exists in the free form, or as a component of membrane inositol phospholipids which are present largely on the inner leaflet of the plasma membrane. Inositol phospholipids, particularly phosphatidylinositol 4,5-bis-phosphate (PIP2), are linked to a number of brain receptor signaling systems including serotonergic, muscarinic, adrenergic, histaminergic, cholecystokinin, tachykinins, metabotropic, neurotensin, platelet activating factor, and others. With receptor stimulation, the signal is transmitted through a series of other proteins. Activation of a GTP-binding protein (Gq), in turn activates plasma membrane phospholipase C releasing the second messenger, myo-inositol 1,4,5-trisphosphate (InsP3), into the cytosol. InsP3 then causes release of free calcium from endoplasmic reticulum into the cytosol, which then activates a number of calcium-sensitive enzymes and receptors. Myo-inositol is made available to the brain through three sources: (i) receptor stimulation (salvage pathway), (ii) de novo production, and (iii) dietary intake. Initial clinical studies have shown that myo-inositol has psychoactive effects, and is effective in the treatment of specific mood and anxiety disorders. Recent preliminary clinical studies have suggested the fascinating possibility that myo-inositol has psychoactive effects, and may be effective in the treatment specific mood and anxiety disorders. Further clinical studies are required using larger groups of patients before definitive conclusions can be drawn upon the use of myo-inositol as a potential psychoactive compound. Myo-inositol as a natural medication increases interest in this newly emerging area of nutritional neuroscience.

The effects of lithium isotopes on the myo-inositol 1-phosphatase reaction in rat brain, liver, and testes

Enzyme inhibition studies were performed with several lithium isotopes in order to more precisely define how lithium inhibits the enzyme myo-inositol 1-phosphatase. This lithium-induced inhibition is thought to be central to the therapeutic effects of lithium in the treatment of manic-depressive disorder. Naturally occurring lithium (NLi) exists as a combination of isotopes: 6Li and 7Li. Lethality studies were performed comparing 6LiCl, 7LiCl, and NLiCl, did not demonstrate a differential effect as previous studies had suggested. Enzyme inhibition studies were performed with these individual lithium isotopes, and compared to the effects of the naturally occurring combination (NLi) on the inhibition of myo-inositol 1-phosphatase using a partially purified enzyme preparation from rat brain, liver and testes. Identical inhibition was observed with all lithium isotopes and their combinations. In addition, both D- and L-myo-inositol 1-phosphates were used as enzyme substrates and found to be equivalent. These experiments, along with previous work demonstrating lithium acting as an uncompetitive inhibitor in the reaction, and the lack of lithium binding sites on the enzyme, suggests the hypothesis that lithium is possibly inhibiting this reaction by interfering with the formation of a transition cyclic intermediate, myo-inositol 1,3-cyclic phosphate, which may be formed from either the D- or L-substrates. This proposal is in contrast to previous suggestions regarding the inhibitory mechanism of action of lithium on the myo-inositol 1-phosphatase reaction.

Human chromosomal localization of a gene for inositol monophosphatase by fluorescence in situ hybridization

INHIBITION of the enzyme inositol monophosphatase (IMPase) (E.C. 3.1.3.25) has been linked to the therapeutic action of lithium in the treatment of manic-depression (bipolar) disorder. Because of the link between bipolar and IMPase, we felt it would be of considerable importance to determine the human chromosomal localization of the IMPase gene. Fluorescence in situ hybridization analysis using a human cDNA clone, which included the 5′-UTR and the complete coding region, mapped the human IMPase gene to chromosome 8q21.2–21.3. No gene locus for manic-depressive disorder has yet been identified. Further studies on this IMPase gene, and other potential gene variants and mutations, should help to determine if specific subgroups of patients with manic-depressive disorder can be determined on a molecular basis, with regard to the IMPase gene.

Specific staining of myo-inositol-1-phosphatase on polyacrylamide gels after electrophoresis*

A sensitive staining method was developed to localise the activity of myo-inositol-1-phosphatase on Polyacrylamide gels after electrophoresis. The method can also be used for non-specific phosphatases as well as for those specific phosphatases acting upon inositol polyphosphates which are prime cellular second messengers. One or two nmol of phosphate is sufficient and less than 3 µg of purified protein will facilitate the localisation of phosphatase. If more phosphatases are present in the enzyme preparation, a combination of inhibitors can be used to suppress the activities of unwanted phosphatases and the use of specific substrates will facilitate the localisation of enzyme of interest.

[13] - Identification of Phosphatidylinositol Trisphosphate in Rat Brain

Publisher Summary This chapter describes the procedures for the identification of phosphatidylinositol trisphosphate (PIP 3 ) in rat brain. Cerebra from several rats are dissected and homogenized initially in a small volume of chloroform/methanol, with several brains pooled into one homogenate, and equal fractions are taken for each sample. During each step of the lipid extraction, the samples are kept under nitrogen using test tubes with Teflon-lined caps. The first acidified chloroform/methanol lipid extract is used for all lipid determinations, as this fraction contains most of the polyphosphoinositides. To separate the inositol lipids silica gel H preoxalated plates were used. The plates are placed in a sealed plexiglass spotting apparatus which allows nitrogen to flow continuously over the plate during spotting. The plates are developed using iodine vapors; the appropriate areas are scraped and scintillation counting is performed. The alkaline hydrolysis of PIP 3 is performed by exposing silica gel to potassium hydroxide. The nonradioactive lipid extract is also added to the mixture as a carrier with perchloric acid to neutralize the mixture with the precipitation of potassium perchlorate. The mixture is centrifuged and analyzed by HPLC. The inositol phosphates are hydrolyzed by acid and alkaline conditions. The phosphate moieties thus liberated from inositol phosphates can be estimated in several ways.

The Use of Ginseng as an Adjunct in Treatment-Resistant Depression

The treatment of depression has evolved over the past several years since the evolution of tricyclic antidepressants (TCAs) to the serotonin selective reuptake inhibitors (SSRIs). However, some patients are resistant to various medications, and various adjunctive medications have been added to the original medication, to promote a therapeutic response. This case report describes a woman, with a long history of treatment-resistant depression, who was treated with a combination of an SSRI and ginseng.