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The Mg 2+ requirements of nonactivated and activated rat liver phosphorylase kinase
Author(s) -
Chrisman Ted D.,
Sobo Gwenn E.,
Exton John H.
Publication year - 1984
Publication title -
febs letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.593
H-Index - 257
eISSN - 1873-3468
pISSN - 0014-5793
DOI - 10.1016/0014-5793(84)80146-5
Subject(s) - enzyme , enzyme activator , phosphorylase kinase , chemistry , glycogen phosphorylase , biochemistry , enzyme assay , kinase
Incubation of rat liver phosphorylase kinase in the presence of MgATP results in a time‐dependent increase in activity, i.e., activation. Determination of the magnitude of activation depends, in large part, on the relative concentrations of Mg 2+ and ATP used in the phosphorylase kinase activity assay, such that as the Mg 2+ to ATP ratio increases less activation is detectable. Prior to activation, maximal activity of nonactivated phosphorylase kinase requires a 2–3 fold molar excess of Mg 2+ (i.e., free Mg 2+ ) over ATP. MgATP‐dependent activation of the enzyme results in an alteration in the free Mg 2+ requirement such that the activity of the activated enzyme is sharply inhibited by the free cation. Inhibition by free Mg 2+ of the activated enzyme is rapidly reversed by removal of free Mg 2+ but is not affected by addition of Ca 2+ . Both nonactivated and activated forms of enzyme appear to be inhibited by free ATP 4– . The results show that the use of high concentrations of free Mg 2+ in the phosphorylase kinase activity assay can blunt or completely obscure changes in enzyme activity following activation of the enzyme.