Changes in histone methylase activity of rat brain and liver with ageing

Abstract

No function has been found for the methylation of the ɛ-N-amino group of lysine in certain histones that have been demonstrated in thymus gland, liver and erythrocytes as well as several other organs¹. It has been suggested, however, that this modification of the histone polypeptide effects the DNA-histone interaction and may be involved in regulating gene expression¹. Methylation appears to be very specific, occurring after histone synthesis is complete². Methylation of certain histones can be correlated with development. Gershey et al. reported that 3-methylhistidine was formed more rapidly in immature erythroid cells than in mature erythrocytes³, and Borun et al. found that in developing HeLa cells the lysyl residues of F_2b, F₃ and F_2a1 histones were methylated mainly in the G2 phase and during mitosis⁴. Methylation of rat brain histones also seems to depend on age. After the intraperitoneal injection of L-¹⁴CH₃-methionine into different age classes of rats we found that only trace amounts of ¹⁴C-methyl were incorporated into brain histones after maturity⁵. If L-³H-lysine and L-¹⁴CH₃-methionine are administered to newborn rats biphasic type decay curves (³H and ¹⁴C) are obtained for all histone subfractions. During brain development (up to 50 d) the arginine-rich histones have a halflife of 32 d while no turnover is observed after maturity as measured by either ³H or ¹⁴C decay. We now find that the level of the histone methylases and the ability of brain histones to accept methyl groups decreases during ageing. Freshly prepared brain and liver nuclei from rats of various ages were incubated with C³H₃-S-adenosylmethionine. The reaction mixtures were chilled to 0° C, the nuclei centrifuged down and extracted three times with 0.14 M NaCl. The residue was washed with methanol–chloroform, acetone, acetic acid (pH 3.0) and the histones extracted with 0.4 M HCl⁶. When L-¹⁴C-lysine was added to the reaction mixture no ¹⁴C was incorporated into the resultant histones excluding in vitro synthesis of histone in our system.