Metabolism of histones of brain and liver.

Turnover of whole histones of brain and liver nuclei was studied after administration of 14C-lysine to adult mice. The radioactivity of histones and other nuclear and cytoplasmic proteins was determined at intervals from 2 hours to 245 days. Two fractions of cerebral histones with halflifetimes of 54 and 104 days and three fractions of hepatic histones with half-lifetimes of 18, 56, and 93 days were found. A larger number of fractions with gradually increasing turnover rates are not excluded. The histones of brain and liver turn over at a lower rate than any of the nuclear and cytoplasmic protein fractions measured in the respective organs. In order to Iabe histones with a very low rate of turnover, 14C-lysine was administered to pregnant mice and the tumover of histones was estimated in their offspring after reaching maturity. In the brain only a slow histone fraction with a half-lifetime of 117 days and in the liver two fractions with half-lifetimes of 58 and 105 days were observed. Over a period from 2 to 84 months in the brain and from 5 to 83 months in the liver the decrease in the specific activity of histones was very small, corresponding to a replacement rate of about 0.6% per day. The turnover rates of histones may reflect division and replacement of various cellular species. The data suggest a metabolic stability of histones which corresponds to that of deoxyribonucleic acid.