The rate of protein degradation in developing brain. Methodological considerations.

Recently we reported that the rate of protein breakdown decreases during development. Breakdown rates were calculated from the rates of protein synthesis and the changes in brain protein content with age. A different study, measuring breakdown by monitoring the loss of label from brain protein after an H14CO3- pulse, came to the opposite conclusion: that the rate of breakdown is low in immature brain and increases during development. We have now investigated some of the factors (the distribution of label in protein and the potential for recycling) that might introduce errors into these measurements. The specific radioactivities of both protein-bound and free amino acids were determined in the brains of young rats several days after an intraperitoneal pulse of H14CO3-. For a number of amino acids the specific radioactivity of the free amino acid is high compared with that of the protein-bound amino acid, and therefore recycling could result in an underestimate of the degradation rate. Because glutamic acid had a relatively low specific-radioactivity ratio, [1-14C]glutamic acid was used in a pulse-labelling experiment to measure degradation. The rate so obtained, 0.6% . h-1, is twice the rate found with H14CO3- labelling (based on total protein-bound radioactivity). Insofar as recycling is a possible complication, 0.6% . h-1 may be a minimum value. Although somewhat higher degradation rates are found after labelling with an intracranial pulse, which was considered as a possible route to limit recycling, there are difficulties in interpreting these data.