title: Regulation of human leukocyte microsomal hydroxymethylglutaryl-CoA reductase activity by a phosphorylation and dephosphorylation mechanism.
abstract: The activity of microsomal 3-hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.34), obtained from cultured human IM-9 lymphoid cells or freshly isolated human peripheral blood leukocytes, is modulated by a phosphorylation/dephosphorylation mechanism. Addition of MgATP + ADP to IM-9 cell microsomal reductase leads to a time-dependent loss of enzyme activity. Inactivated reductase is reactivated by rat liver reductase phosphatase. Kinase-dependent IM-9 cell microsomal reductase, prepared by heating IM-9 microsomes for 15 min at 50 degrees C, is inactivated in the presence of MgATP and ADP only after addition of cytosolic reductase kinase from either IM-9 cells, freshly isolated leukocytes or rat liver. Inactivation is time-dependent and dependent on the cytosolic protein concentration. Inactivated reductase is reactivated by rat liver reductase phosphatase. For cultured IM-9 cells and freshly isolated leukocytes incubated with culture medium for 2 h, the ratios of active (unphosphorylated) to total (phosphorylated + unphosphorylated) reductase activity are 0.22 and 0.43, respectively. Thus, in addition to its regulation by changes in the amount of total enzyme protein, human leukocyte reductase activity is also modulated by a phosphorylation/dephosphorylation mechanism.
mesh_terms: D000227:Adenine Nucleotides / Q000378:metabolism; D003600:Cytosol / Q000201:enzymology; D006358:Hot Temperature; D006801:Humans; D006903:Hydroxymethylglutaryl CoA Reductases / Q000378:metabolism*; D007962:Leukocytes / Q000201:enzymology* / Q000648:ultrastructure; D008274:Magnesium / Q000494:pharmacology; D008861:Microsomes / Q000201:enzymology; D010766:Phosphorylation
chemical_list: D000227:Adenine Nucleotides; D006903:Hydroxymethylglutaryl CoA Reductases; D008274:Magnesium