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The production of an exogenous protein by the transfection of a plasmid DNA encoding the protein was kinetically analyzed, to determine the efficiency of the transfection. Cultured NIH3T3 or HeLa cells, and the luciferase protein were used as a model system in this experiment. The findings indicate that at least a 8 × 104- and 4 × 103-fold molar amounts of luciferase protein was produced from one copy of the plasmid DNA molecule in NIH3T3 and HeLa cells, respectively. The rate of elimination of luciferase activity upon DNA transfection was smaller than that for the luciferase protein itself (kel for DNA transfection < kel for the luciferase protein), suggesting that a decrease in intranuclear active DNA was the main determinant of the elimination rate in this case. A preliminary pharmacokinetic model is proposed, based on the results obtained.