Metabolism of inositol bis-, tris-, tetrakis- and pentakis-phosphates in GH3 cells

Abstract

Previous studies demonstrated a multiplicity of isomers of inositol phosphates in GH3 rat pituitary tumour cells. In order to determine their origin, we have investigated the metabolism of radiolabelled inositol phosphates (IPn) in GH3 cell homogenates by using h.p.l.c. I(1,4,5)P3 is either phosphorylated to I(1,3,4,5)P4 (in the presence of ATP) or dephosphorylated to I(1,4)P2 (in the absence of ATP). I(1,4)P2 is dephosphorylated to I(4)P (greater than 95%). I(1,3,4,5)P4 hydrolysis yields two products. By using dual-labelled [32P, 3H]I(1,3,4,5)P4 with 32P in either the 3 or the 4/5 position, we have identified the probable configuration of these isomers. The predominant (greater than 97%) IP3 formed is I(1,3,4)P3, with a minor I(1,4,5)P3 peak. Subsequent I(1,3,4)P3 hydrolysis yields two IP2 isomers [the major (approximately equal to 85%) is I(3,4)P2; the minor (approximately equal to 15%) is I(1,3)P2] and two IP isomers (the major (approximately equal to 90%) is I(3)P [L-I(1)P], the minor I(4)P). IP5 is very slowly dephosphorylated to and IP4 of undetermined isomeric configuration. We have also examined GH3 cell lipids for the presence of phosphoinositides either more highly phosphorylated than PIP2 (as potential sources of the IP4/IP5 and IP6 in these cells) or phosphorylated in positions other than 1, 4 and 5, and have been unable to find evidence of either. These data suggest two main routes of metabolism for I(1,4,5)P3 in GH3 cells: either (1) phosphorylation to I(1,3,4,5)P4, and the subsequent consecutive dephosphorylation to I(1,3,4)P3, I(3,4)P2 and finally L-I(1)P [D-I(3)P]; or (2) dephosphorylation to I(1,4)P2 and, subsequently, I(4)P.