Electron Microscopical Localization of the α2-Macroglobulin Thiol Ester Sites

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Abstract
The active thiol ester groups of .alpha.2- macroglobulin (.alpha.2M) were reacted with a biotin derivative and the sites labelled with avidin-ferritin complexes. Elecron micrographs show a strong preference of attachment of the ferritins to the ends of the rods of the H-shaped moelcules. A mutual "cross-labelling" was observed in an .alpha.2M preparation which yielded dimers of the molecules which must have been forming during purification. The molecules were mostly attached to each other at the ends of the rods of the H-shaped moelcules. It is concluded that the thiol esters responsible for the covalent attachment of the proteinases (and other moelcules) may be located more in the distal parts of the .alpha.2M moelcules, while the proteinase molecules are finally trapped near to the centre of the .alpha.2M molecules.