Baculovirus-Mediated Expression of Human 65 kDa and 67 kDa Glutamic Acid Decarboxylases in SF9 Insect Cells and Their Relevance in Diagnosis of Insulin-Dependent Diabetes Mellitus

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cDNAs coding for the full-length human 65 and 67 kDa glutamic acid decarboxylases (GAD₆₅ and GAD₆₇) were amplified from pancreas and hippocampus cDNA libraries by polymerase chain reaction, respectively. Both cDNAs were inserted into a baculovirus vector which mediated highly efficient expression of the human GAD₆₅ and GAD₆₇ with histidine-hexapeptides as affinity ligands at their C-termini in Spodoptera frugiperda (Sf9) cells. The recombinant GAD proteins were purified to homogeneity by affinity chromatography using a metal-chelating matrix. The infected Sf9 insect cells expressed the recombinant human GAD₆₅ and GAD₆₇ with natural-like conformations, as confirmed by measurement of their enzyme activities as well as their fully restored autoantigenicities. Immunoprecipitation of metabolically labeled infected Sf9 cells demonstrated the autoantigenic potential of the recombinant GAD proteins. The practicability of using recombinant GAD₆₅ and GAD₆₇ derived from the baculovirus expression system for the development of an immunoassay for the diagnosis of insulin-dependent diabetes mellitus is discussed.