Diurnal variation in oxidation of L-[1-(14)C]leucine and L-[1-(14)C]lysine in rats.

Diurnal variation in the oxidation of L-[1-(14C]leucine and L-[1-(14C]lysine was estimated in rats that had access to feed for 8 out of 24 hours. The labeled amino acids were given by single injection, after which more than 50% of the total 14CO2 expired in 6.25 hours was collected in the first 45 minutes. For both labeled amino acids maximum rates of 14CO2 expiration were observed 2 hours after the beginning of the feeding period, and minimum rates were observed 2 hours before the feeding period. The ratios of peak to nadir for L-[1-(14C]leucine and L-[1-(14C]lysine were 4.4 and 11, respectively. We found that measurements made 2 and 6 hours after the end of the feeding period represented the mean daily rate of oxidation for both labeled amino acids. This time period seems appropriate for making measurements of quantitative amino acid catabolism when extrapolation to daily rates is required.