Reaction of formaldehyde with calf-thymus nucleohistone

The reactions of formaldehyde with calf thymus nucleohistone were analyzed in the following ways: measurement with fluorescamine of the decrease in primary amino groups resulting from hydroxymethylation and crosslinking reactions, measurement with dodecylsulphate-gel electrophoresis of formation of histone oligomers, measurement of fixation of histones to the DNA in nucleohistone, and measurement of changes in the circular dichroism spectrum in the region of 250--300 nm. In the presence of formaldehyde, the primary amino groups of histones decreased very rapidly, attaining an equilibrium within 60 min, and successively intermolecular crosslinks were also formed between histone molecules, the resulting dimers and oligomers being separable by dodecylsulfate-gel electrophoresis. Whereas the fixation reaction proceeded much more slowly. The extent of fixation could be measured more accurately by dodecylsulfate/sucrose centrifugation analysis than by sulfuric acid extraction. After removal of formaldehyde from the reaction mixture, the fraction of masked amino groups decreased, perhaps due to the reverse reaction, but the extent of fixation of histones continued to increase with time. No specificity was observed among five molecular species of histones in the fixation reaction. With increase in formaldehyde concentration, the ellipticity of nucleohistone decreased to a minimum with about 0.4% formaldehyde, and then increased.