A sensitive radioisotopic assay of myo-inositol: Its application to rat pancreatic islets

A radioisotopic procedure for the assay of myo-inositol is presented. It is based on the generation of NADH from NAD⁺ in the reaction catalyzed by myo-inositol dehydrogenase and the subsequent NADH-dependent conversion of 2-[U-¹⁴C]ketoglutarate to ¹⁴C-labeled l-glutamate in the reaction catalyzed by glutamate dehydrogenase. This method was applied to the measurement of myo-inositol in rat pancreatic islets. The myo-inositol islet content was decreased when the animals were fed a diet deprived of myo-inositol. When incubated in the absence of exogenous d-glucose, pancreatic islets, like parotid cells, released myo-inositol in the incubation medium. Over 90 min of incubation, a rise in extracellular d-glucose concentration increased the myo-inositol islet content, which was decreased, however, after incubation in the presence of carbamylcholine. These findings indicate that the myo-inositol content of islets is affected by nutritional and other environmental factors.     

Studies on the developmental pattern of the enzymes converting glucose 6-phosphate to myo-inositol in the rat

The myo-inositol level of plasma was determined during pre- and postnatal development of the rat. Fetal concentrations exceeded those of maternal rats by nearly 10-fold. Immediately after birth, the myo-inositol level decreased but was maintained at values 3–4 times that of the lactating dams. The cyclitol content of rat milk was high and rose during lactation to a maximum of 1.6 mM.The biosynthesis of myo-inositol from glucose 6-phosphate is catalyzed by glucose 6-phosphate:l-myo-inositol-1-phosphate cyclase and l-myo-inositol-1-phosphate phosphatase. The activity of both enzymes was monitored in fetal and neonatal liver, maternal liver, placenta, and mammary gland. Results indicated that the fetal liver accounted for over 48% of the total carcass cyclase and 26% of the total carcass phosphatase activity. Developmental changes correlated well with the pattern of myo-inositol in fetal rat plasma. Similarly, the enzymes of the myo-inositol biosynthetic pathway increased in rat mammary gland in close agreement with the myo-inositol content of milk and diminished to prelactation activities within 24 hr after the onset of involution.The myo-inositol level of colostrum and milk of five human subjects was highest (2.8 mM) before birth and decreased to 40% of that level 5 days postpartum, where it remained for at least 3 weeks. Even after 7 months of lactation, the milk of one subject contained 3–4-fold more myo-inositol than all commercial infant formulas analyzed.     

Identification of 6-O-alpha-D-mannopyranosyl myo-inositol from Saccharomyces cerevisiae

A mannosyl inositol isolated from Baker's yeast was shown to be α-linked from the 1 position of mannose to the 6 position of myo-inositol by comparison of the products of permethylation, hydrolysis, and reduction of the disaccharide. The structure was established using chemical and enzymatic methods, gas chromatography, and combined gas chromatographymass spectrometry.     

Improving d-glucaric acid production from myo-inositol in E. coli by increasing MIOX stability and myo-inositol transport

d-glucaric acid has been explored for a myriad of potential uses, including biopolymer production and cancer treatment. A biosynthetic route to produce d-glucaric acid from glucose has been constructed in Escherichia coli (Moon et al., 2009b), and analysis of the pathway revealed myo-inositol oxygenase (MIOX) to be the least active enzyme. To increase pathway productivity, we explored protein fusion tags for increased MIOX solubility and directed evolution for increased MIOX activity. An N-terminal SUMO fusion to MIOX resulted in a 75% increase in d-glucaric acid production from myo-inositol. While our directed evolution efforts did not yield an improved MIOX variant, our screen isolated a 941 bp DNA fragment whose expression led to increased myo-inositol transport and a 65% increase in d-glucaric acid production from myo-inositol. Overall, we report the production of up to 4.85 g/L of d-glucaric acid from 10.8 g/L myo-inositol in recombinant E. coli.     

Studies on avian erythrocyte metabolism: purification and properties of myo-inositol 1-phosphatase form chick erythrocytes

An enzyme capable of hydrolyzing myo-inositol 1-phosphate was identified and partially purified from the erythrocytes of 7-day chicks. It has an apparent molecular weight of approximately 60,000, is heat stable, and has a pH of optimal activity between 6.5 and 7.3. In most regards the kinetic properties are similar to the myo-inositol 1-phosphatases of rat testis, rat mammary gland, bovine brain, and of yeast. The enzyme has an absolute requirement for a divalent cation; Mg²⁺ gave the greatest activity, with an optimal concentration of 2.5 mm in the standard assay employed. Zn²⁺, Co²⁺, and Mn²⁺ supported activity to a lesser degree. Activity was inhibited by NaF, HgCl₂, and p-hydroxymercuribenzoate. myo-Inositol tetrakis (dihydrogen phosphate) and myo-inositol 1,3,4,5,6-pentakis (dihydrogen phosphate) were not substrates for this enzyme and inhibited the hydrolysis of myo-inositol 1-phosphate. Unlike other phosphatases for myo-inositol 1-phosphate, this enzyme cleaved myo-inositol 1-phosphate (K_m = 8.6 × 10^?5 m) and myo-inositol 2-phosphate (K_m = 2.86 × 10^?4 m) at approximately the same rates. It also hydrolyzed 2′-purine and pyrimidine ribonucleotides about as well as myo-inositol 1-phosphate, but was only 20–30% as active against the 3′-ribonucleotides and had scarcely any activity against the 5′-ribonucleotides. The amount of enzyme activity in erythrocytes of embryos, chicks, and mature chickens was the same (~29 μmol/ml rbc/h). The biological function of this enzyme in avian erythrocytes is unclear at this time. Other tissues containing this phosphatase also have an enzyme which synthesizes myo-inositol 1-phosphate from glucose 6-phosphate, but we have been unable to detect the presence of such an enzyme in avian erythrocytes.     

Dietary phytase and myo-inositol supplementation are associated with distinct plasma metabolome profile in broiler chickens

Phytase enzyme is used as a dietary supplement in broiler nutrition to improve phosphorous bioavailability. Phytase deliberates phosphate groups from phytic acid and produces myo-inositol after total dephosphorylation. Myo-inositol is a bioactive compound having beneficial modulatory effects on metabolism in humans. However, it is not well understood if and how phytic acid degradation products, particularly myo-inositol, can modulate metabolism in broiler chicken. The purpose of this study was to investigate effects of dietary supplements of phytase and myo-inositol on the blood plasma metabolome profile of broiler chickens. Broilers were provided a nutrient-adequate control diet or the same diet supplemented with either 3.5 g myo-inositol or 500, 1500 or 3000 units of phytase, per kilogram of feed (grower diet). Broilers were group-housed in floor pens (eight pens per diet) and provided one of the treatment diets for 22 days. Then, blood was collected from one bird per pen, resulting in eight replicated measurements per diet. A targeted metabolomics approach was applied to the heparin plasma. Body weight of the birds was not significantly affected by the treatments. Plasma myo-inositol concentrations were significantly increased by myo-inositol supplementation and phytase supplementation at 500 and 1500 units/kg. Metabolites generally affected by phytase supplementation belonged to the groups of acyl-carnitines, phosphatidylcholines, sphingomyelins, lysophosphatidylcholine, biogenic amines and amino acids. Compared to the control diet, phytase supplements had significantly higher plasma concentrations of kynurenine and creatinine, but lower concentrations of histamine and cis-4-hydroxyproline. Myo-inositol supplementation significantly increased plasma concentrations of dopamine and serotonine. While some metabolites were similarly affected by myo-inositol and phytase supplementation, others were distinctly differently affected. We conclude that myo-inositol, either as a directly added supplement or indirectly released from phytate upon phytase supplementation, can affect specific metabolic pathways. Additional effects found on phytase supplementation may be related to intermediary phytate degradation products. Results are indicative for innovative hypothesis to be tested in future experiments, for instance, with regard to relationships between phytase or myo-inositol supplements and bird immunity or behaviour.     

Biosynthesis of myo-inositol in rat mammary gland. Isolation and properties of the enzymes

myo-Inositol 1-phosphate synthase (EC 5.5.1.4) and 1l-myo-inositol 1-phosphatase (EC 3.1.3.25) were isolated and partially purified from lactating rat mammary gland. The synthase had an apparent molecular weight of 290,000 as determined by gel filtration; its pH optimum was 7.2, and the K_m for glucose 6-phosphate was 0.5 mm. No other compound could act as a substrate, but the synthase was inhibited 100% by d-gluconic acid 6-phosphate, 54% by d-fructose 6-phosphate, 31.8% by d-galactose 6-phosphate, and 29.6% by d-mannose 6-phosphate each at 5mm. Activity was stimulated 2-fold by the addition of 1 mm NAD⁺ and 40% by 14 mm ammonium ions, whereas it was inhibited by 30% in the presence of 1 mm NADH and by 93.6% when incubated with 1 mmp-mercuribenzoate. Reagents which interfere with Schiff-base formation, pyridoxal 5′-phosphate and trinitrobenzenesulfonate, inhibited the enzyme, but EDTA was without effect.The 1l-myo-inositol 1-phosphatase from rat mammary tissue appears to exist in a native tetrameric form of 210,000 as determined by gel filtration which, upon heating at 70 °C for 15 min, is converted into a stable monomer of approximately 52,000. Mg²⁺ (1.5 mm) was an absolute requirement for activity though Mn²⁺ gave 17% of the activity provided by Mg²⁺. Sodium, potassium, or ammonium ions were stimulatory, but lithium ions were strongly inhibitory. 1l-myo-Inositol 1-phosphatase specifically cleaved 1l-myo-inositol 1-phosphate and was 60% as active toward l-α-glycerol phosphate with only minor activity toward other phosphorylated compounds. The pH optimum was 8.0 and the K_m for 1l-myo-inositol 1-phosphate was 0.8 mm.     

Membrane lipid biosynthesis in Chlamydomonas reinhardtii. Partial characterization of CDP-diacylglycerol: myo-inositol 3-phosphatidyltransferase

Myo-inositol may be incorporated in the formation of phosphatidylinositol by two mechanisms. One reaction utilizes CDP-diacylglycerol and is catalyzed by phosphatidylinositol (PtdIns) synthase (CDP-diacylglycerol: myo-inositol 3-phosphatidyltransferase, EC 2.7.8.11). The second reaction is the phosphatidylinositol: myo-inositol exchange reaction, in which a free inositol is exchanged for an existing inositol headgroup. This characterization of inositol incorporation into phosphatidylinositol in the green alga Chlamydomonas reinhardtii provides evidence for the presence of both reactions. The transferase reaction required a divalent cation and exhibited its maximum activity at 2.0 mM Mn²⁺. The optimal pH for this reaction was 8.5–9.0. The best substrate concentrations were 0.5 mM CDP-diacylglycerol and 1.2 mM myo-inositol, with an estimated K_m for myo-inositol of 0.2 mM. The exchange reaction also required Mn²⁺ for activity, but became saturated at 0.5 mM Mn²⁺. The optimal pH of the exchange reaction was 8.0, the optimal myo-inositol concentration was 0.3 mM, and the estimated K_m for myo-inositol in this reaction was 0.015 mM. Measurement of the transferase reaction in cell fractions of C. reinhardtii indicated that the activity occurred primarily in the microsomal fraction, with little or no activity in the plastids.     

Anaerobic peroxisomes in Entamoeba histolytica metabolize myo-inositol

Entamoeba histolytica is believed to be devoid of peroxisomes, like most anaerobic protists. In this work, we provided the first evidence that peroxisomes are present in E. histolytica, although only seven proteins responsible for peroxisome biogenesis (peroxins) were identified (Pex1, Pex6, Pex5, Pex11, Pex14, Pex16, and Pex19). Targeting matrix proteins to peroxisomes is reduced to the PTS1-dependent pathway mediated via the soluble Pex5 receptor, while the PTS2 receptor Pex7 is absent. Immunofluorescence microscopy showed that peroxisomal markers (Pex5, Pex14, Pex16, Pex19) are present in vesicles distinct from mitosomes, the endoplasmic reticulum, and the endosome/phagosome system, except Pex11, which has dual localization in peroxisomes and mitosomes. Immunoelectron microscopy revealed that Pex14 localized to vesicles of approximately 90–100 nm in diameter. Proteomic analyses of affinity-purified peroxisomes and in silico PTS1 predictions provided datasets of 655 and 56 peroxisomal candidates, respectively; however, only six proteins were shared by both datasets, including myo-inositol dehydrogenase (myo-IDH). Peroxisomal NAD-dependent myo-IDH appeared to be a dimeric enzyme with high affinity to myo-inositol (Km 0.044 mM) and can utilize also scyllo-inositol, D-glucose and D-xylose as substrates. Phylogenetic analyses revealed that orthologs of myo-IDH with PTS1 are present in E. dispar, E. nutalli and E. moshkovskii but not in E. invadens, and form a monophyletic clade of mostly peroxisomal orthologs with free-living Mastigamoeba balamuthi and Pelomyxa schiedti. The presence of peroxisomes in E. histolytica and other archamoebae breaks the paradigm of peroxisome absence in anaerobes and provides a new potential target for the development of antiparasitic drugs.     

Distribution of myo-inositol dehydrogenase in algae

The enzyme myo-inositol dehydrogenase (InDH; EC 1.1.1.18) catalyses the NAD-dependent oxidation of myo-inositol to scyllo-inosose (2-keto-inositol). A survey within different algal groups showed that this enzyme is present in rhodophytes, glaucocystophytes, phaeophytes, xanthophytes and haptophytes but not in green algae, euglenophytes and chrysophytes. Anion-exchange chromatography of crude homogenates resulted in two distinct peaks of activity. Both isoenzymes were specific for myo-inositol and scyllo-inosose. epi- and scyllo-inositol as well as epi-inosose were only poor substrates, while all other polyols and sugars tested did not serve as substrates. A possible role of the InDH isoenzymes is the shuttling of reducing power between the mitochondrion and the cytosol.     

Production of glucaric acid from myo-inositol in engineered Pichia pastoris

A potential myo-inositol oxygenase (ppMIOX) was identified as a functional enzyme and a glucaric acid synthetic pathway was firstly constructed in Pichia pastoris. Coexpression of the native ppMIOX and the urinate dehydrogenase (Udh) from Pseudomonas putida KT2440 led to obvious accumulation of glucaric acid (90.46 ± 0.04 mg/L) from myo-inositol whereas no glucaric acid was detected from glucose. In comparison, coexpression of the heterologous mouse MIOX (mMIOX) and Udh resulted in higher titers of glucaric acid from glucose and myo-inositol, 107.19 ± 11.91 mg/L and 785.4 ± 1.41 mg/L, respectively. By applying a fusion expression strategy with flexible peptides, the mMIOX specific activity and the glucaric acid concentration were significantly increased. Using glucose and myo-inositol as carbon substrates, the production of glucaric acid was substantially enhanced to 6.61 ± 0.30 g/L in fed-batch cultures. To the best of our knowledge, this is the highest reported value to date.     

Formation of a series of myo-inositol phosphates during growth of rice plant cells in suspension culture

A series of myo-inositol phosphates including myo-inositol mono-to hexa-phosphates was observed during growth of cultured riceplant cells. We also found that ³²Pi and myo-[2-³H] inositolwere incorporated into all these myo-inositol phosphates. myo-Inositolphosphorylating activity, which depended on ATP and Mg²⁺, wasdetected in the soluble fraction from the cells, and the reactionproduct was identified as myo-inositol-2-phosphate. (Received January 21, 1980; )     

Use of Per-C-Deuterated myo-Inositol for Study of Cell Wall Synthesis in Germinating Beans

Cell wall polysaccharides of the hypocotyl and roots in germinating beans (Phaseolus vulgaris L.) were selectively labeled in arabinosyl, xylosyl, and galacturonosyl residues by per-C-deuterated myo-inositol, which was introduced through 72 hours of imbibition. Glucuronate residues remained unlabeled. Selected ion gas chromatography-mass spectrometry analysis revealed that deuterium was not redistributed in these three sugar residues or into other carbohydrate residues during this conversion, suggesting that the labeled residues are formed exclusively via the myo-inositol oxidation pathway and that no glucogenesis from myo-inositol takes place during this conversion. The presence of a significant level of deuterated arabinose, xylose, and galacturonate after just 72 hours of imbibitional uptake of per-C-deuterated myo-inositol indicated that the myo-inositol oxidation pathway has a predominant role in the biosynthesis of new cell walls.     

Phosphorylation of myo-inositol in ripening grains of rice and wheat. Incorporationof phosphate-32P and myo-inositol-3H into myo-inositol phosphates

To elucidate the mechanism of the phosphorylation of myo-inositolin the process of phytate formation, feeding experiments oforthophosphate-³²P and myo-inositol-³H in the ripening grainsof rice and wheat were performed. It was found that ³²P and³H were incorporated into myo-inositol mono- and hexa-phosphates.The same results were obtained when a mixture of "cold" myo-inositolpolyphosphates was administered to the grains before feedingphosphate-³²P. Based on these results it is concluded that phosphorylationof free myo-myo-inositol in the formation of phytate does nottake place in a stepwise fashion but may proceed through anunknown myo-inositol derivative. (Received August 2, 1967; )     

Metabolic engineering of Corynebacterium glutamicum for production of scyllo-inositol,a drug candidate against Alzheimer's disease

Scyllo-inositol has been identified as a potential drug for the treatment of Alzheimer's disease. Therefore, cost-efficient processes for the production of this compound are desirable. In this study, we analyzed and engineered Corynebacterium glutamicum with the aim to develop competitive scyllo-inositol producer strains. Initial studies revealed that C. glutamicum naturally produces scyllo-inositol when cultured with myo-inositol as carbon source. The conversion involves NAD⁺-dependent oxidation of myo-inositol to 2-keto-myo-inositol followed by NADPH-dependent reduction to scyllo-inositol. Use of myo-inositol for biomass formation was prevented by deletion of a cluster of 16 genes involved in myo-inositol catabolism (strain MB001(DE3)Δiol1). Deletion of a second cluster of four genes (oxiC-cg3390-oxiD-oxiE) related to inositol metabolism prevented conversion of 2-keto-myo-inositol to undesired products causing brown coloration (strain MB001(DE3)Δiol1Δiol2). The two chassis strains were used for plasmid-based overproduction of myo-inositol dehydrogenase (IolG) and scyllo-inositol dehydrogenase (IolW). In BHI medium containing glucose and myo-inositol, a complete conversion of the consumed myo-inositol into scyllo-inositol was achieved with the Δiol1Δiol2 strain. To enable scyllo-inositol production from cheap carbon sources, myo-inositol 1-phosphate synthase (Ino1) and myo-inositol 1-phosphatase (ImpA), which convert glucose 6-phosphate into myo-inositol, were overproduced in addition to IolG and IolW using plasmid pSI. Strain MB001(DE3)Δiol1Δiol2 (pSI) produced 1.8 g/L scyllo-inositol from 20 g/L glucose and even 4.4 g/L scyllo-inositol from 20 g/L sucrose within 72 h. Our results demonstrate that C. glutamicum is an attractive host for the biotechnological production of scyllo-inositol and potentially further myo-inositol-derived products.     

Trypanosoma brucei Bloodstream Forms Depend upon Uptake of myo-Inositol for Golgi Complex Phosphatidylinositol Synthesis and Normal Cell Growth

myo-Inositol is a building block for all inositol-containing phospholipids in eukaryotes. It can be synthesized de novo from glucose-6-phosphate in the cytosol and endoplasmic reticulum. Alternatively, it can be taken up from the environment via Na⁺- or H⁺-linked myo-inositol transporters. While Na⁺-coupled myo-inositol transporters are found exclusively in the plasma membrane, H⁺-linked myo-inositol transporters are detected in intracellular organelles. In Trypanosoma brucei, the causative agent of human African sleeping sickness, myo-inositol metabolism is compartmentalized. De novo-synthesized myo-inositol is used for glycosylphosphatidylinositol production in the endoplasmic reticulum, whereas the myo-inositol taken up from the environment is used for bulk phosphatidylinositol synthesis in the Golgi complex. We now provide evidence that the Golgi complex-localized T. brucei H⁺-linked myo-inositol transporter (TbHMIT) is essential in bloodstream-form T. brucei. Downregulation of TbHMIT expression by RNA interference blocked phosphatidylinositol production and inhibited growth of parasites in culture. Characterization of the transporter in a heterologous expression system demonstrated a remarkable selectivity of TbHMIT for myo-inositol. It tolerates only a single modification on the inositol ring, such as the removal of a hydroxyl group or the inversion of stereochemistry at a single hydroxyl group relative to myo-inositol.     

myo-Inositol transport in plasma membrane of rat kidney

The myo-inositol transport system in kidney plasma mambrane preparation was investigated. myo-Inisitol uptake was more rapid than that due to non-specific uptake. Specific myo-inisitol uptake was temperature dependent and pH sensitive; the optimum was at pH 7.4. Specific myo-insitol uptake was inhibited by scyllitol and inosose-2 but not(+)-inositol, d-glucose, d-galactose or mannitol. Inhibition of myo-inositol uptake by scyllitol was of the competitive type. It showed that the transport system is stereospecific and that myo-inositol shares the transport system with scyllitol. Moreover, the specific myo-inositol uptake was inhibited by phlorizin. Counter transport of myo-inositol was demonstrated. The results indicate that myo-inositol uptake by the membrane preparation represents the entry into the intravesicular spaces rather than binding to the membrane.It was concluded that the plasma membrane of rat kidney has a cyclitol carrier system specific to myo-inositol and scyllitol.     

Role of myo-inositol in the synthesis of phosphatidylglycerol and phosphatidylinositol in the lung

The addition of $\text{myo}$-inositol to lung microsomes inhibited phosphatidylglycerol synthesis up to 94% while it stimulated that of phosphatidylinositol. The inhibition was evident only when CDP-diacylglyceride availability was limiting the rate of acidic phospholipid synthesis. Excess $\text{myo}$-inositol given to rabbits for two days decreased surfactant phosphatidylglycerol from 5.3–5.7% to 0.4–0.5%, and increased that of phosphatidylinositol from 5.4–5.8% to 9.3–8.6% of total phospholipid. The composition of other surfactant phospholipids as well as those in mitochondria and microsomes were little affected. The quality of microsomally synthesized acidic phospholipids may be controlled by $\text{myo}$-inositol at the biosynthetic surface.     

Purification and characterization of myo-inositol 6-O-methyltransferase from Vigna umbellata Ohwi et Ohashi

The cyclitol 1d-4-O-methyl-myo-inositol (d-ononitol) is accumulated in certain legumes in response to abiotic stresses. S-Adenosyl-l-methionine:myo-inositol 6-O-methyltransferase (m6OMT), the enzyme which catalyses the synthesis of d-ononitol, was extracted from stems of Vigna umbellata Ohwi et Ohashi and purified to apparent homogeneity by a combination of conventional chromatographic techniques and by affinity chromatography on immobilized S-adenosyl-l-homocysteine (SAH). The purified m6OMT was photoaffinity labelled with S-adenosyl-l-[¹⁴C-methyl]methionine. The native molecular weight was determined to be 106 kDa, with a subunit molecular weight of 40 kDa. Substrate-saturation kinetics of m6OMT for myo-inositol and S-adenosyl-l-methionine (SAM) were Michaelis-Menten type with K _m values of 2.92 mM and 63 M, respectively. The SAH competitively inhibited the enzyme with respect to SAM (K _i of 1.63 M). The enzyme did not require divalent cations for activity, but was strongly inhibited by Mn²⁺, Zn²⁺ and Cu²⁺ and sulfhydryl group inhibitors. The purified m6OMT was found to be highly specific for the 6-hydroxyl group of myo-inositol and showed no activity on other naturally occurring isomeric inositols and inositol O-methyl-ethers. Neither d-ononitol, nor d-3-O-methyl-chiro-inositol, d-1-O-methyl-muco-inositol or d-chiro-inositol (end products of the biosynthetic pathway in which m6OMT catalyses the first step), inhibited the activity of the enzyme.Abbreviations DTT dithiothreitol - m6OMT myo-inositol 6-O-methyltransferase - SAH S-adenosyl-l-homocysteine - SAM S-adenosyl-l-methionine We are greatful to Professor M. Popp (University of Vienna) for helpful discussion and comment. This work was supported by Grant P09595-BIO from the Austrian Science Foundation (FWF).     

A defect of the myo-inositol maintenance mechanism in the lens of hereditary cataract mice

The myo-inositol uptake system was studied in lenses of normal and hereditary cataract mouse. The normal mouse was able to accumulate myo-inositol continuously from medium and keep it in a high concentration. The specific myo-inositol uptake was dependent on temperature and it decreased in Ca²⁺-free medium. In contrast, specific uptake of myo-inositol reached a plateau after 15 min in the cataract mouse lens although initial incorporation was more rapid than that in normal mouse lens. This uptake system was not affected by temperature or Ca²⁺ in the medium. The rate of myo-inositol efflux into the medium was more rapid in the cataract lens than that of the normal lens. It was shown that the low level of myo-inositol in the lens of hereditary cataract mouse was due to the defect of myo-inositol transport system and the enhanced efflux rate. These results suggest a dysfunction of the lens membrane.