Electron microscopy of papain crystals

Crystalline suspensions of the proteolytic enzyme papain have been studied by negative staining with sodium phosphotungstate and uranyl acetate, and by thin sectioning. The needle-like crystals of papain exhibit pronounced longitudinal and transverse periodicities and are clearly shown to be formed by the coalescence of parallel bundles of individual fibrils. Optical diffractometry performed on suitable regions of the electron micrographs containing papain crystals have been employed to produce the optical transform for subsequent image filtering. Transverse thin sections of the thicker crystals reveals a complex honeycomb organization with pronounced aqueous channels. This organization is held to be responsible for the varying images of the negatively stained crystals, which result from the varying orientations of the crystal axis relative to the electron beam. It is proposed that papain may represent another useful protein for studying electron microscopically the fibre-to-crystal transformation process, as well as for the understanding of the formation of varying crystalline states.