Firefly luciferase has been widely used as a model substrate to study folding (1–3) and renaturation (4–6) of protein because of its rapid and sensitive bloluminescenct activity. Although normally localized in peroxisomes, luciferase folds to the native state on expression in bacteria, in mammahan cells In culture (7), and in rabbit reticulocyte lysate (RRL) (1–3). RRL also efficiently facilitates the renaturation of thermally (5–6) or chemically (1, 4, 8—10) denatured luciferase. Efficient luciferase renaturation requires optimal ATP, Mg²⁺, and K⁺ concentrations (5). Thermal denaturation of luciferase produces unfolded intermediates that mimic denatured protein produced in a cell subjected to heat stress, whereas chemical denaturation of luclferase with guani-diniumHCl was thought to generate unfolded luciferase, which closely mimics nascent unfolded luciferase. However, recent results indicate that the folding pathway of chemically denatured luciferase is not identical to that followed by newly synthesized luciferase (1).