Protein catabolism in bean leaf discs: accumulation of a soluble fragment of ribulose-1,5-bisphosphate carboxylase/oxygenase under oxygen deficiency

Discs isolated from the primary leaves of 2-week-old bean plants were used to study the influence of different light intensities (between 0 and 27 mu E m(-2) s(-1) on Rubisco and chlorophyll catabolism. In discs with a water-infiltrated intercellular space, the rate of chlorophyll degradation strongly depended on the light intensity, while illumination was far less effective in non-infiltrated leaf discs. It has been suggested in the past that stromal proteins can be cleaved in the intact organelle. In our experiments, a 45 kDa Rubisco breakdown product accumulated in infiltrated leaf discs in darkness (where chlorophyll was stable) or after an incubation under N-2. One or several steps involved in the further degradation of this catabolite might be oxygen- or energy-dependent. The solubility of Rubisco and of the fragment accumulating under hypoxia in the dark was investigated, since an association of this enzyme with membranes under certain conditions has been reported in the past. As judged from SDS-PAGE gels, neither the intact large subunit nor the fragment accumulated in the membrane fraction in our experimental system. The intermediates formed during Rubisco degradation might depend on environmental conditions. At present, the steps involved in the catabolism of this enzyme cannot be generalized.