Microbial amino acid synthesis and utilization in rats: incorporation of N from NH4Cl 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 15 N incorporated into body lysine in four germ-free ( 15 N-GF) and four conventional ( 15 N-CV) rats. They were fed for 10 d on a protein-free diet containing fermentable carbohydrates and 15 NH 4 Cl ; another four conventional rats (control), fed on the same diet but with unlabelled NH 4 Cl, were used to estimate the natural abundance of 15 N. The eviscerated carcass of each rat was homogenized and a sample hydrolysed. Lysine was isolated by ion-exchange chromatography and its 15 N enrichment was measured by isotope-ratio mass spectrometry. The 15 N-CV rats significantly incorporated 15 N into their body lysine. The 15 N-GF rats had a statistically significant, although small, incorporation of 15 N into their body lysine, probably arising from a measurement artifact. It was concluded, therefore, that all | 15 N|lysine was of microbial origin. The total lysine content in the body and the 15 N enrichment of lysine in the microbial fraction of the faeces of the 15 N-CV rats were also determined. The amount of microbial lysine absorbed by the 15 N-CV rats was estimated by dividing the total amount of | 15 N|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 .