Investigation on the chromium oxide interaction with soluble chromatin and histone H1: a spectroscopic study. 
Chromatin has been introduced as a tool for studying heavy metals action in nuclei.  Chromium oxide is highly soluble and toxic with chronic exposure leading to mutagenesis and carcinogenesis.  In the present study, for the first time, the binding affinity of chromium oxide to rat liver chromatin and histone proteins was investigated.  Reduction of chromatin absorbencies at 210 and 260 nm (hypochromicity) and fluorescence emission intensity upon metal binding represented quenching of the metal with chromatin chromophores.  Binding isotherms demonstrated a positive cooperative binding pattern revealing higher affinity of the metal to chromatin compared to DNA as confirmed by the binding constants.  Melting temperature of chromatin was altered in a dose dependent manner and suggests partial removal of histones from the chromatin at metal concentrations higher than 15 μg/ml.  Chromium oxide decreased the absorbance of histone H1 at 210 nm (hypochromicity) and fluorescence emission intensity revealed quenching of the metal with tyrosine residue located in the core domain of H1.  Also the interaction of chromium oxide with histone H1 increased its secondary structures.  The results suggest toxic effect of very low concentrations of chromium oxide on chromatin and in this reaction both DNA and histones are involved.