Amino acid incorporation by preparations from the developing rat brain.

1. A study has been made of the ability of cerebral microsome-cell sap systems, taken from rats at various ages, to incorporate [(14)C]valine. 2. The systems appear to follow Michaelis-Menten kinetics when cell sap is considered as substrate, microsomes as enzyme and total counts/mg. of microsomal protein as a measure of reaction velocity. 3. Reproducible ;affinity constant' values are obtained, and the system from 4-day-old rats has a higher V(max.) and affinity constant than the system from adult rats. 4. It is suggested the the amino acid-incorporating ability of different systems may be compared by this means.