Purification and characterization of recombinant histidine-tagged human platelet 12-lipoxygenase expressed in a baculovirus/insect cell system.

A baculoviral expression vector consisting of a sequence encoding a six-histidine tag apposed to the human platelet 12-lipoxygenase cDNA, under control of the polyhedrin promoter, was constructed. Recombinant 12-lipoxygenase baculoviruses were used to infect Spodoptera frugiperda insect cells (Sf9). At 54 h post-infection, maximal 12-lipoxygenase activity and protein levels were achieved: the enzyme was purified to apparent homogeneity in a single step by nickel-ion-chelation chromatography in which the (His)6-tagged 12-lipoxygenase was eluted with 100 mM imidazole. The purified enzyme metabolized arachidonic acid almost exclusively to 12-hydroperoxyeicosatetracnoic acid with little, if any. epoxyalcohol or reduction products and had a Vmax of 2–4 μmol min-1mg protein-1, Km, of 10 μM and keat of ≈250 min-1. Linoleic acid, on the other hand, was converted to (13S)-13-hydroperoxy-octadecadienoic acid at a rate which was about 2% of that obtained with arachidonic acid as substrate, but displayed the same Km. The enzyme was most active between pH 7.5–8 and activity was stimulated significantly in the presence of 0.006% Tween-20. A polyclonal antibody to the recombinant enzyme was generated and found to recognize a single 75-kDa band in platelets, human erythroleukemia cells and 12-1ipoxygenase baculoviral-infected Sf9 cells by immunoblot and immunoprecipitation methods. 12-Lipoxygenase protein represented 0.1% of the total soluble protein in platelet preparations. In immunofluorescence experiments 12-lipoxygenase was observed in the cytoplasm of infected insect cells and in the human megakaryoblastic DAMI cell line. The isolation of large quantities of pure human platelet 12-1ipoxygenase should facilitate detailed biochemical structure/function studies. [ABSTRACT FROM AUTHOR]