Multiple forms of the large subunit of wheat ribulose bisphosphate carboxylase generated by excess iodoacetamide

An analysis of the subunits of d-ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) of wheat (Triticum aestivum) L. cv. Falcon) by gel isoelectric focusing in 8 M urea revealed one type of large subunit and one type of small subunit. This observation contrasts with consistent reports in the literature that this enzyme has three types of large subunit. Carbamidomethylation of the enzyme before isoelectric focusing with a 300-fold molar excess of iodoacetamide over protein thiol groups resulted in three bands of large subunit. Preparative procedures involving aggregation of the enzyme also resulted in complex isoelectric focusing patterns. The simplest patterns, with one type of large subunit and one type of small subunit, were obtained when the enzyme was isolated rapidly and gently by immunoprecipitation or preparative polyacrylamide gel electrophoresis, and analysed by isoelectric focusing without alkylation of thiol groups. The native enzyme probably contains no charge diversity in the large subunit.