Abstract
The proportions of endogenous protein nitrogen (N), peptide N and free amino acid N in digesta N from the distal ileum of the rat were determined immediately after collection or following storage frozen ( โ 20ยฐC and โ 196ยฐC). Eighteen growing rats were given a proteinโfree diet for 6 days and then euthanased. Samples of digesta were then collected from the terminal 20 cm of ileum. The digesta samples were centrifuged and the supernate was ultrafiltered. Total nitrogen and total amino acids were determined in the precipitate plus retentate (MW> 10 000 Da) and ultrafiltrate (MW < 10 000 Da) fractions, and urea, creatinine, ammonium and free amino acids in the ultrafiltrate. The storage of digesta did not significantly affect the proportions of Nโcontaining substances in the precipitate plus retentate or ultrafiltrate excepting that ammonium levels in the latter fraction were significantly lower for digesta processed immediately. On average, 67% of the total digesta N was in the precipitate plus retentate fraction and 61% of the total N was protein N. Essential and nonโessential amino acid N comprised 26 and 36%, respectively, of total digesta N in the precipitate plus retentate. On average, the ultrafiltrate fraction contained 33% of the total digesta N, and the total digesta N contained 10.4, 10.6, 1, 1.5, 1.5 and 0.5 g kg^โ1^ free amino acid N, peptide N, urea N, ammonium N and creatinine N, respectively. Nonamino N (urea N + ammonium N + creatinine N) contributed 4% of the total N in the whole digesta. The N from essential and nonโessential amino acids in the ultrafiltrate (free + peptide), expressed as percentages of the total N in the whole digesta, were 8 and 13 %, respectively. On average, 21 % of the total digesta N was present as free amino acid N and peptide N. These findings give support to the application of a proposed method for the determination of total endogenous ileal amino acid excretion under conditions of peptide alimentation.