Manganese-stimulated phosphatidylinositol headgroup exchange in rat liver microsomes.
Manganese-dependent, CMP-independent incorporation of myo-[3H]inositol into phospholipids of rat liver microsomes was studied in an attempt to clarify the physiological significance of this headgroup-exchange reaction. The enzyme responsible worked best with Mn2+ as a co-factor, but Mg2+ at physiological concentrations supported a significant rate of incorporation. The K(m) for myo-inositol was around 11 microM, yet incorporation of myo-[3H]inositol was unaffected by as much as 5 mM choline, ethanolamine, glycerol or serine; as this is a reversible reaction, these data imply that phosphatidylinositol is the most likely lipid substrate. Similarly, other inositols showed an apparent affinity at least two orders of magnitude lower than myo-inositol. Glucosamine alpha 1-6 myo-inositol also had a low affinity for the enzyme, making it unlikely that this headgroup-exchange activity is part of a metabolic pathway for glycosyl phosphatidylinositols. The phosphatidylinositol radiolabelled by headgroup exchange was deacylated and deglycerated, and the resulting inositol phosphate headgroup cochromatographed on anion exchange HPLC with myo-inositol l-phosphate. The simplest interpretation of all the data is the apparent paradox that this enzyme functions at a slow rate under physiological conditions to remove the myo-inositol headgroup from phosphatidylinositol, only to replace it with another myo-inositol.
Irvine
R F
RF
Department of Pharmacology, University of Cambridge, UK. rfi20@cam.ac.uk
D000818:Animals; D017261:Glycosylphosphatidylinositols / Q000096:biosynthesis; D007294:Inositol / Q000737:chemistry; D008345:Manganese / Q000494:pharmacology*; D008862:Microsomes, Liver / Q000187:drug effects* / Q000378:metabolism; D010716:Phosphatidylinositols / Q000737:chemistry* / Q000378:metabolism*; D051381:Rats; D013379:Substrate Specificity
D017261:Glycosylphosphatidylinositols; D010716:Phosphatidylinositols; D008345:Manganese; D007294:Inositol