Kinase Assay: MLCK assay; MLCK activity is assayed with peptide substrate (KKRPQRATSNVFS-NH2) or myosin light chain. The peptide substrate (24 μM) is phosphorylated in a reaction mixture containing 25 mM Tris-HC1 (pH 7.5), 0.5 mg/mL bovine serum albumin, 4 mM MgCl2, 0.5 mM CaCl2, 2.6 nM calmodulin, 1.5 nM MLCK, and 400 μM ATP in a final volume of 0.25 mL. After a 10-min preincubation at 28 oC without ATP, the reaction is started by addition of ATP at 28 oC and terminated by the addition of 0.1 mL of 10% (v/v) acetic acid after 30 min. The mixture is analyzed by high performance liquid chromatography: column, Unisil Pack 5C18 4.6 X 150 mm; solvent, 18% (v/v) acetonitrile, 0.1% (v/v) trifluoroacetic acid in water; flow rate, 1.0 mL/min; temperature, 40 oC; detection, absorbance at 220 nm. Percent of reaction is calculated from the ratio of peak areas of phosphorylated form to those of unphosphorylated form. Specific activity measured under the conditions described above is 0.81 μmol/min/mg. Myosin light chain (108 μg/mL) is phosphorylated in a reaction mixture containing 25 mM Tris-HC1 (pH 7.5), 0.5 mg/mL bovine serum albumin, 4 mM MgCl2, 0.5 mM CaCl2, 4.2 nM calmodulin, 0.92 nM enzyme, and 10 μM[γ-32P]ATP (100-900 cpm/pmol) in a final volume of 0.25 mL. After a 3-min preincubation at 30 oC without ATP, the reaction is started by the addition of [γ-32P]ATP at 30 oC and stopped by the addition of 0.125 mL of trichloroacetic acid after 5 min. The acid-precipitable materials are collected on a nitrocellulose membrane filter and washed with four 1-mL aliquots of 5% (v/v) trichloroacetic acid. The radioactivity on the filter is measured in a toluene scintillation fluid, using a Packard Tri-Carb liquid scintillation spectrometer Model 4530. The specific activity measured under the conditions is 1.23 μmol/min/mg.

Cell Assay: The inhibition of MLCK by Wortmannin is not affected by calmodulin or peptide substrat, while reduced by high concentration of ATP. Wortmannin directly interacts with the catalytic domain of MLCK and leads to an irreversible loss of the enzyme activity. Wortmannin has no inhibitory to cAMP-dependent protein kinase, cGMP-dependent protein kinase, and calmodulin-dependent protein kinase II, and has little effect on protein kinase C activity. Wortmannin inhibits N-formylmethionyl-leucylphenylalanine (fMLP)-stimulated PtdInsP3 (phosphatidylinositol 3,4,5-trisphosphate) formation with IC50 of 5 nM and this inhibition is completely abolished when pretreated with 100 nM Wortmannin in human neutrophils, with increased PtdInsP2 levels and no effects on cellular PtdInsP and PtdIns contents. Wortmannin could develop oscillatory changes in F-actin content and does not inhibit fMLP-stimulated actin polymerization in neutrophils. Wortmannin irreversibly inhibits phosphatidylinositol 3-kinase (PI3-kinase) activity with binding to the 110-kDa protein (IC50 of 3 nM) and has no effect PI4-kinase in RBL-2H3 cells. Wortmannin also inhibits both Fc epsilon RI-mediated histamine secretion and leukotriene release, with no effect on the activation of the tyrosine kinase Lyn. Wortmannin completely abolishes the insulin-induced hexose uptake in isolated rat adipocytes at 0.1 μM, without impairing isoproterenol-stimulated lipolytic activity. Wortmannin suppresses insulin-induced production of nitric oxide by 50% at 500 nM in human umbilical vein endothelial cells, which is in response to IGF-1.

[1] J Biol Chem. 1992 Feb 5;267(4):2157-63. [2] Biochem J. 1993 Dec 1;296 ( Pt 2):297-301.