The pure cinnamomin A-chain is unstable compared to that in the mixture of A- and B-chain or in intact cinnamomin molecule either being stored at 4degreesC or being heated. When being heated at 45degreesC for 20 min, the A-chain generates partially unfolded intermediate and loses its tertiary structure as monitored by circular dichroism.(CD) and tryptophan fluorescence, thus resulting in the inactivity of its RNA N-glycosidase albeit it retains most of its secondary structures. This partially unfolded intermediate is sensitive to protease, exhibiting property of a molten globule. The changes in conformation and activity are irreversible upon cooling. The partially unfolded intermediate can fully restore its RNA N-glycosidase activity in the presence of cinnamomin B-chain. The phenomenon, that the cinnamomin B-chain mediates the refolding of partially unfolded A-chain, probably plays an important role in the intracellular transport of the cytotoxic protein, i.e., keeping the structural stability of A-chain and refolding partially unfolded A-chain that occasionally appeared in the process of intracellular transport, to avoid the destiny of proteolysis that occurs in most denatured proteins in cell. (C) 2003 Elsevier Science (USA). All rights reserved.