Microbial amino acid synthesis and utilization in rats: Incorporation of N-15 from (NH4Cl)-N-15 into lysine in the tissues of germ-free and conventional rats

The absorption of lysine synthesised by the gastrointestinal microflora was estimated by comparing the N-15 incorporated into body lysine in four germ-free (N-15-GF) and four conventional (N-15-CV) rats. They were fed for 10 d on a protein-free diet containing fermentable carbohydrates and (NH4Cl)-N-15; another four conventional rats (control), fed on the same diet but with unlabelled NH4Cl, were used to estimate the natural abundance of N-15. Th, eviscerated carcass of each rat was homogenized and a sample hydrolysed. Lysine was isolated by ion-exchange chromatography and its N-15 enrichment was measured by isotope-ratio mass spectrometry. The N-15-CV rats significantly incorporated N-15 into their body lysine. The N-15-GF rats had a statistically significant, although small, incorporation of N-15 into their body lysine, probably arising from a measurement artifact. It was concluded, therefore, that all [N-15]lysine was of microbial origin. The total lysine content in the body and the N-15 enrichment of lysine in the microbial fraction of the faeces of the N-15-CV rats were also determined. The amount of microbial lysine absorbed by the N-15-CV rats was estimated by dividing the total amount of [N-15]lysine in the body by the enrichment of microbial lysine. It was estimated that the daily absorption of microbial lysine by the conventional rats was 21.3 (SE 2.04) mg/kg body weight(0.75).