Keratin was extracted from purified cornified cells of newborn rats in Tris-HCl-buffered 8 M urea containing beta-mercaptoethanol. Microfilaments were assembled in vitro by reducing the ionic strength of buffer and the urea concentration. One millimolar concentration of KCl and NaCl did not affect filament formation, but the same concentration of divalent cations greatly altered this process. CaCl2 and MgCl2 induced gelation of keratin by formation of bundles of birefringent macrofilaments. ZnCl2, CuSO4 and HgCl2 formed greater numbers of macrofilaments and the protein aggregated.
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