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 microflor a 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 w ere fed for 10 d on a protein-free diet containing fermentable carbohy drates 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 ho mogenized 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 in to their body lysine. The N-15-GF rats had a statistically significant , although small, incorporation of N-15 into their body lysine, probab ly arising from a measurement artifact. It was concluded, therefore, t hat 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 fracti on 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 divi ding the total amount of [N-15]lysine in the body by the enrichment of microbial lysine. It was estimated that the daily absorption of micro bial lysine by the conventional rats was 21.3 (SE 2.04) mg/kg body wei ght(0.75).