The baculovirus (AcNPV)-insect cell expression system was employed for the production of Escherichia coli fl-galactosidase as a model heterologous protein. Preliminary experiments made clear that most of all Spodoptera cells were adapted well to IPL-41 medium supplemented with 10% fetal bovine serum. Sf9 cells provided a maximum cell density of 4.9 X 10^6 cells/ml, while the E. coli β-galactosidase expression level of Sf9 cells infected with recombinant baculovirus (Ac360-501 β-gal) was found to be less than that of Sf21 cells. The Ac360-501 β-gal-infected Sf21 cells expressed E. coli β-galactosidase at a high level, and the amount of 120 kDa-proteins corresponding to β-galactosidase was densitometrically estimated to be 4.1 mg per 5.0 x 10^5 cells in a milliliter of culture broth, representing more than 36% of total cellular protein. The β-galactosidase activity expressed by infected Sf21 cells grown in IPL-41 medium was 1.4 X 10^5 units per 5.0 X 10^5 cells, and the enzyme expression level was equivalent to 7 times of that expressed by Sf9 cells under the same conditions.
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