Identification ofchiro-Inositol and Its Formation by Isomerization ofmyo-Inositol during Hydrolysis of Glycosylphosphatidylinositol-Anchored Proteins

myo-Inositol has been believed to be a sole inositol isomer existing in phosphatidylinositol (PI) and related derivatives. In this experiment,chiro-inositol, an inositol isomer other thanmyo-inositol, was identified in hydrolytic products from several GPI-anchored proteins. Thechiro-inositol contents in several different GPI-anchored proteins including 5′-nucleotidase of bovine liver and alkaline phosphatase of mouse NS-1 varied with hydrolytic conditions of these GPI anchor. Isomerization of 20–60% ofmyo-inositol occurred on the hydrolysis in 6nHCl solution. Under the hydrolytic conditions of a HCl gas stream in place of solution, however, isomerization was very low (less than 0.1%). Even in the hydrolysis under HCl gas stream, existence of CNBr accelerated the isomerization of inositol in GPI up to 70–95%. In the hydrolysis of phosphatidylinositol ormyo-inositol 1-phosphate, however, a significant amount ofchiro-inositol was not detected in 6nHCl solution or in the existence of CNBr under the HCl stream. These facts indicated that isomerization occurred during the hydrolysis of the GPI anchor, whenmyo-inositol is substituted by glucosamine at 6-OH and is substituted by phosphate at 1-OH. It also suggested that the former identification ofchiro-inositol in GPI structure in the various reports might be due to isomerization.