Relationship of Ribulose-1,5-bisphosphate Carboxylase-Oxygenase Specific Activity to Subunit Composition

Ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBPCase, EC 4.1.1.39) was isolated from Nicotiana sylvestris and from two cultivars and three nuclear substitution lines of Nicotiana tabacum. Isoelectric focusing patterns, supported by amino acid analyses and tryptic peptide mapping, were used to divide these enzymes into two categories: (a) RuBPCase with variable large subunits and identical small subunits; and (b) RuBPCase with identical large but different small subunits. Specific activities for both the carboxylation and oxygenation reactions were determined for all six RuBPCase enzymes under standard conditions of activation and assay. High, intermediate, and low levels of carboxylase (880, 530, and 340 nanomoles HCO₃⁻ per milligram per minute) and oxygenase (66, 45, and 35 nanomoles O₂ per milligram per minute) activity were noted. The carboxylase to oxygenase ratios ranged from 9 to 14.