Histones have been isolated from the mammary glands of rats and rabbits and chromatographically separated into very lysine-rich, slightly lysine-rich, and arginine-rich classes. When these histones were compared with corresponding fractions obtained from rabbit thymus, substantial differences in amino acid composition were observed for the very lysine-rich and arginine-rich classes. In the case of the arginine-rich class the difference was shown to arise from nonhistone contaminants and could be largely eliminated by changing the method used to extract histones from deoxyribonucleoprotein of mammary gland. The very lysine-rich histone from mammary gland approached that of thymus in amino acid composition when minor histone fractions were removed with 5% trichloracetic acid. The very lysine-rich histone from mammary gland was subfractionated by chromatography on Amberlite IRC-50 with a shallow guanidinium chloride gradient. The pattern of subfractions differed for rabbit thymus and rabbit mammary gland, in agreement with results reported previously. In addition mammary glands of rabbits and rats differed in their complement of very lysine-rich histones. Mammary gland histones from animals in different stages of pregnancy and lactation were compared to see whether the histone complement changed with the changing function of the gland. The three major histone classes were not observed to change significantly from late pregnancy to early lactation in rats. The complement of very lysine-rich histones also remained essentially constant in that period. Very lysine-rich histones from the mammary glands of rabbits were compared over the wider range of mammary development from nonpregnant to lactating to involuting states. Although the very lysine-rich histone patterns from rabbits in late pregnancy and lactation were similar, they differed from those of the nonpregnant and the involuting states.
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Stellwagen et al. (1968) studied this question.