Age-dependent utilization of phenylalanine for the synthesis of neuronal and glial proteins.

1. Groups of 5-, 10-, 18- and 43-day old rats were injected intracerebrally with a single dose of [U14C] phenylalanine and its conversion to acid-soluble metabolites was determined in the cerebral cortex over a period of 24 h. Although the most rapid utilization of [14C] phenylalanine occurred initially in the oldest group, by 45 min the 18-day old cortex had the lowest levels of unchanged [14C] phenylalanine; by 24 h [14C] phenylalanine was, in turn, lowest in the 10-day old cortex. 2. The specific radioactivity of [14C] phenylalanine decreased rapidly at all ages for the first 2 h; it then continued to decrease in the two younger age groups while it remained virtually constant in the two older ones. 3. We also determined the incorporation of [14C] phenylalanine into, and the loss of radioactivity from, cortical, glial and neuronal proteins, isolated from animals injected on their 10th, 18th and 43rd day of life and followed over a period of two weeks. a. The proteins of the 10-day old cortex attained the highest specific radioactivity and were labeled at the highest rate. b. The glial proteins and particularly those of the 10-day old cortex were consistently more highly radioactive than the neuronal proteins; the rates of radioactivity decay from neuronal and glial proteins were parallel, however. c. The decay of the radioactivity followed a similar time course at all ages and for all proteins examined, exhibiting two phases, a rapid one extending from day 1 (day 3 in the 18-day old cortex) to day 7 and a slower phase extending through the entire second week.