inositol

Inositol or cyclohexane-1,2,3,4,5,6-hexol is a chemical compound with formula C6H12O6 or (-CHOH-)6, a sixfold alcohol (polyol) of cyclohexane.

It exists in nine possible stereoisomers, of which the most prominent form, widely occurring in nature, is cis-1,2,3,5-trans-4,6-cyclohexanehexol, or myo-inositol (former name meso-inositol).

Inositol is a carbohydrate, though not a classical sugar. It is almost tasteless, with a small amount of sweetness.

Isomers and structure

The isomer myo-inositol is a meso compound that possesses an optically inactive plane of symmetry through the molecule, and meso-inositol is an obsolete name that refers to myo-inositol. Besides myo-inositol, the other naturally occurring stereoisomers (though in minimal quantities) are scyllo-, muco-, D-chiro-, and neo-inositol.

The other possible isomers are L-chiro-, allo-, epi-, and cis-inositol. As their name denotes, the two chiro inositols are the only pair of inositol enantiomers, but they are enantiomers of each other, not of myo-inositol.

In its most stable conformational geometry, the myo-inositol isomer assumes the chair conformation, which puts the maximum number of hydroxyls to the equatorial position, where they are farthest apart from each other.

In this conformation the natural myo isomer has a structure in which five of the six hydroxyls (the 1st, 3rd, 4th, 5th, and 6th are equatorial, whereas the 2nd hydroxyl group is axial.

Myo-inositol

The isomer myo-inositol is a meso compound that possesses an optically inactive plane of symmetry through the molecule, and meso-inositol is an obsolete name that refers to myo-inositol. Besides myo-inositol, the other naturally occurring stereoisomers (though in minimal quantities) are scyllo-, muco-, D-chiro-, and neo-inositol.

Myo-inositol plays an important role as the structural basis for a number of secondary messengers in eukaryotic cells, including inositol phosphates, phosphatidylinositol (PI) and phosphatidylinositol phosphate (PIP) lipids.

Inositol or its phosphates and associated lipids are found in many foods, in particular fruit, especially cantaloupe and oranges.

In plants, the hexaphosphate of inositol, phytic acid or its salts, the phytates, are found. Phytic acid occurs also in cereals with high bran content and also nuts and beans, but inositol as phytate is not directly bioavailable to humans in the diet, since it is not digestible (some food preparation techniques partly break down phytates to change this—see phytic acid for details).

Inositol as it occurs in certain plant-derived substances such as lecithins, however, is well-absorbed and relatively bioavailable.

Myo-inositol was once considered as a member of the vitamin B complex, however, because it is produced by the human body from glucose, it is not an essential nutrient.

Some substances such as niacin can also be synthesized in the body, but are not made in amounts considered adequate for good health, and thus are still classified as essential nutrients. However, there is no convincing evidence that this is the case for myo-inositol.

Synthesis

Myo-Inositol is synthesized from glucose-6-phosphate (G-6-P) in two steps. First, G-6-P is isomerised by ISYNA1 to myo-inositol 1-phosphate, which is then dephosphorylated by IMPase 1 to give free myo-inositol.

In humans most inositol is synthesized in the kidneys, in typical amounts of a few grams per day.

Signaling

Inositol and a number of its mono and polyphosphates function as the basis for a number of signaling and secondary messenger molecules. They are involved in a number of biological processes, including:

• Insulin signal transduction
• Cytoskeleton assembly
• Nerve guidance (Epsin)
• Intracellular calcium (Ca2+) concentration control
• Cell membrane potential maintenance
• Serotonin activity modulation
• Breakdown of fats and reducing blood cholesterol
• Gene expression