myo-Inositol oxygenase: molecular cloning and expression of a unique enzyme that oxidizes myo-inositol and d-chiro-inositol

Abstract

myo-Inositol oxygenase (MIOX) catalyses the first committed step in the only pathway of myo-inositol catabolism, which occurs predominantly in the kidney. The enzyme is a non-haem-iron enzyme that catalyses the ring cleavage of myo-inositol with the incorporation of a single atom of oxygen. A full-length cDNA was isolated from a pig kidney library with an open reading frame of 849bp and a corresponding protein subunit molecular mass of 32.7kDa. The cDNA was expressed in a bacterial pET expression system and an active recombinant MIOX was purified from bacterial lysates to electrophoretic homogeneity. The purified enzyme displayed the same catalytic properties as the native enzyme with Km and kcat values of 5.9mM and 11min−1 respectively. The pI was estimated to be 4.5. Preincubation with 1mM Fe2+ and 2mM cysteine was essential for the enzyme's activity. d-chiro-Inositol, a myo-inositol isomer, is a substrate for the recombinant MIOX with an estimated Km of 33.5mM. Both myo-inositol and d-chiro-inositol have been implicated in the pathogenesis of diabetes. Thus an understanding of the regulation of MIOX expression clearly represents a potential window on the aetiology of diabetes as well as on the control of various intracellular phosphoinositides and key signalling pathways.