Electrophoretic Assay for Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase in Guard Cells and Other Leaf Cells of Vicia faba L

The ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) contents of guard cells and other cells of Vicia faba L. leaflet were determined. To prevent proteolysis, proteins of frozen protoplast preparations or of cells excised from freeze-dried leaf were extracted directly in a sodium-dodecyl-sulfate-containing solution, which was heated immediately after sample addition. Protein profiles of the different cell types were obtained by electrophoresis of the extracts and subsequent densitometry of the stained protein bands. About one-third of the protein of palisade parenchyma and of spongy parenchyma was Rubisco large subunit. Using chlorophyll (Chl):protein ratios previously obtained, we calculate mesophyll contained ca. 22 millimoles Rubisco per mole Chl. In contrast, guard-cell protoplast preparations were calculated to contain from 0.7 to 2.2 millimoles Rubisco per mole Chl. The upper end of this range is an overestimate resulting from contamination by mesophyll and to the method of peak integration. Extracts of excised guard cells were calculated to contain 0.05 to 0.17 millimole Rubisco per mole Chl. We conclude that Rubisco is absent, or virtually so, in guard cells of V. faba.