EVIDENCE FOR LONG-LIVED COMPONENTS IN DEVELOPING MOUSE TISSUES LABELED WITH LEUCINE

1. 1. Four mice were injected at 15 days of age with 14C leucine; 10-22 weeks later they were injected with 3H leucine. After the lapse of an additional 4-7 weeks, the animals were sacrificed and ten of their tissues were fractionated into mitochondria, microsomes, supernatant, nuclear histones, globulins and residues. These fractions were counted, and ratios of the two labels obtained. 2. 2. High ratios of 14C to 3H counts indicated the presence of long-lived components synthesized early in life; such substances were consistently found as prominent components in all fractions obtained from brain and muscles, but occured only occasionally in fractions obtained from other tissues. 3. 3. Analysis of brain and muscle fractions established that the long-lived components contain proteins. However, in the case of the brain, lipids derived from leucine label were also among such fractions. 4. 4. The relationships of long-lived protein components to developmental and ageing processes are discussed. © 1969.