The Large Subunit of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase is Fragmented into 37-kDa and 16-kDa Polypeptides by Active Oxygen in the Lysates of Chloroplasts from Primary Leaves of Wheat

Lysates of chloroplasts isolated from wheat (Triticum aestivum L. cv. Aoba) leaves were incubated on ice (pH 5.7) for 0 to 60 min in light (15 μnol quanta m^-2 s^-1), and degradation of the large subunit (LSU) of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco: EC 4.1.1.39) was analyzed by applying immunoblotting with site-specific antibodies against the N-terminal, internal, and C-terminal amino acid sequences of the LSU of wheat Rubisco. The most dominant product of the breakdown of the LSU and that which was first to appear was an apparent molecular mass of 37-kDa fragment containing the N-terminal region of the LSU. A 16-kDa fragment containing the C-terminal region of the LSU was concomitantly seen. This fragmentation of the LSU was inhibited in the presence of EDTA or 1,10-phenanthroline. The addition of active oxygen scavengers, catalase (for H₂O₂) and n-propyl gallate (for hydroxyl radical) to the lysates also inhibited the fragmentation. When the purified Rubisco from wheat leaves was exposed to a hydroxyl radical-generating system comprising H₂O₂, FeSO₄ and ascorbic acid, the LSU was degraded in the same manner as observed in the chloroplast lysates. The results suggest that the large subunit of Rubisco was directly degraded to the 37-kDa fragment containing the N-terminal region and the 16-kDa fragment containing the C-terminal region of the LSU by active oxygen, probably the hydroxyl radical, generated in the lysates of chloroplasts.