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 mu mol quanta m(-2) s(-1)), and degradation of the large subunit (L SU) of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco: EC 4. 1.1.39) was analyzed by applying immunoblotting with site-specific ant ibodies against the N-terminal, internal, and C-terminal amino acid se quences 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 apparen t molecular mass of 37-kDa fragment containing the N-terminal region o f the LSU, A 16-kDa fragment containing the C-terminal region of the L SU was concomitantly seen, This fragmentation of the LSU was inhibited in the presence of EDTA or 1,10-phenanthroline, The addition of activ e oxygen scavengers, catalase (for H2O2) and n-propyl gallate (for hyd roxyl radical) to the lysates also inhibited the fragmentation. When t he purified Rubisco from wheat leaves was exposed to a hydroxyl radica l-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 d egraded to the 37-kDa fragment containing the N-terminal region and th e 16-kDa fragment containing the C-terminal region of the LSU by activ e oxygen, probably the hydroxyl radical, generated in the lysates of c hloroplasts.