A cyclic alcohol, C₆H₁₂O₆, that is a constituent of certain cell phosphoglycerides; the most important isomer is myo-inositol (see formula). It is sometimes classified as a member of the vitamin B complex but it can be synthesized by many animals and it is not regarded as an essential nutrient in humans. Phosphatidylinositol, a constituent of plasma membranes, is a precursor of the intracellular second messenger molecules, inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG); these are produced, through the mediation of phospholipase C, in response to the binding of substances, e.g. serotonin, to their receptors on the cell surface. IP₃ interacts with receptors in the endoplasmic reticulum, causing the release of stored calcium ions into the cytoplasm. Calcium channels in the plasma membrane are also opened, and the marked rise in intracellular calcium triggers the activation of various signal pathways, via the calcium-binding protein calmodulin. DAG stays near the plasma membrane, where it helps to activate members of the protein kinase C family, which in turn activate their respective signal pathways. These pathways mediate such cellular events as smooth muscle contraction, adrenaline secretion, and histamine secretion.

Myo-inositol