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Phosphatidylinositol: myo ‐Inositol Exchange Activity in Intact Nerve Endings: Substrate and Cofactor Dependence, Nucleotide Specificity, and Effect on Synaptosomal Handling of myo ‐Inositol
Author(s) -
Berry Gerard,
Yandrasitz John R.,
Cipriano Victoria M.,
Hwang Shing Mei,
Segal Stanton
Publication year - 1986
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1986.tb00620.x
Subject(s) - inositol , phosphatidylinositol , cofactor , free nerve ending , nucleotide , chemistry , substrate (aquarium) , synaptosome , biochemistry , microbiology and biotechnology , biophysics , biology , anatomy , enzyme , signal transduction , membrane , receptor , gene , ecology
Micromolar concentrations of CMP produced a large increase in Mn 2+ ‐dependent phosphatidylinositol: myo ‐inositol exchange activity in isolated nerve endings or synaptosomes. The apparent K m for CMP was 2 μ M , and that for myo ‐inositol was 38 μ M. Only cytidine nucleotides were capable of enhancing activity, and this effect is probably specific for CMP, because the synaptosomal preparation rapidly converted CTP or CDP to CMP. Manganese did not affect the uptake of myo ‐inositol into the synaptosomal cytosolic fraction or myo ‐inositol levels. Determinations of myo ‐inositol specific activity showed that the Mn 2+ ‐enhanced labeling of phosphatidylinositol was not accompanied by a decrease of label content in free myo ‐inositol. This lack of an effect on intrasynaptosomal myo ‐inositol and the dependence of exchange on cytidine nucleotides whereas cytidine itself was previously found to be without effect show that for the bulk of Mn 2+ ‐dependent exchange activity, it is the myo ‐inositol in the incubation medium that is being directly incorporated into membrane‐bound phosphatidylinositol. Because CMP dependence is the hallmark of exchange catalyzed by CDP‐diacylglycerol:inositol phosphatidyl transferase, this enzyme is likely to be responsible for most of the exchange activity in synaptosomes. The strong affinity of this exchange system for CMP suggests that endogenous levels of this nucleotide might support Mn 2+ ‐dependent exchange in the absence of added nucleotide.