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Details on the sample preparation
Selectively stained by injection of horseradish peroxidase, embedded in Spurr's resin and cut into 2-3 um thick sections.
Enzyme Preparations. S. cerevisiae PDC was purified to near homogeneity from baker's yeast by modification of a published procedure. Highly purified E1 was obtained by resolution of PDC with 2 M NaCl at pH 7.3 followed by FPLC on a Superdex 200 column. The weight-average molecular weight of the PDC was determined by light scattering measurement to be ~8 x 106. On the basis of the known molecular weight of the complex and its component enzymes and the experimentally determined polypeptide chain ratios of E2/BP/E3, we estimated that the subunit composition of the S. cerevisiae PDC is ~24 E1 tetramers, 60 E2 monomers, 12 BP monomers, and 8 E3 dimers. Sufficient E1 was added to a sample of the PDC preparation to increase the molar ratio of E1/E2 core to 60:1. This product is designated larger PDC or ~60 E1/E2 core PDC
embedment in vitreous ice.
Detergent-solubilized particles eluted from the cation-exchange column were directly adsorbed for 1 min to parlodion carbon-coated copper grids rendered hydrophilic by glow discharge at low pressure in air. Grids were washed with 4 drops of double-distilled water and stained with 2 drops of 0.75% uranyl formate. Images were recorded on Eastman Kodak Co. SO-163 sheet film with a Hitachi H-7000 electron microscope operated at 100 kV. Electron micrographs of single particles adsorbed to the carbon film were digitized using a Leafscan-45 scanner (Leaf Systems, Inc., Westborough, MA).