Microbial amino acid synthesis and utilization in rats: incorporation of15N from15NH4Cl into lysine in the tissues of germ-free and conventional rats

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Abstract
The absorption of lysine synthesised by the gastrointestinal microflora was estimated by comparing the15N incorporated into body lysine in four germ-free (15N-GF) and four conventional (15N-CV) rats. They were fed for 10d on a protein-free diet containing fermentable carbohydrates and15NHM4Cl; another four conventional rats (control), fed on the same diet but with unlabelled NH4Cl, were used to estimate the natural abundance of15N. The eviscerated carcass of each rat was homogenized and a sample hydrolysed. Lysine was isolated by ion-exchange chromatography and its15N enrichment was measured by isotope-ratio mass spectrometry. The15N-CV rats significantly incorporated15N into their body lysine. The15N-GF rats had a statistically significant, although small, incorporation of15body lysine, probably arising from a measurement artifact. It was concluded, therefore, that all [15N]lysine was of microbial origin. The total lysine content in the body and the15N enrichment of lysine in the microbial fraction of the faeces of the15N-CV rats were also determined. The amount of microbial lysine absorbed by the15N-CV rats was estimated by dividing the total amount of [15N]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 weight0·75