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Am J Physiol Endocrinol Metab 277: E597-E607, 1999;
0193-1849/99 $5.00
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Vol. 277, Issue 4, E597-E607, October 1999

Availability of intestinal microbial lysine for whole body lysine homeostasis in human subjects

Cornelia C. Metges1,2, Antoine E. El-Khoury1, Lidewij Henneman3, Klaus J. Petzke2, Ian Grant3, Shahinaze Bedri1, Paulo P. Pereira1, Alfred M. Ajami4, Malcolm F. Fuller3, and Vernon R. Young1

1 Laboratory of Human Nutrition, School of Science and Clinical Research Center, Massachusetts Institute of Technology, Cambridge 02139; 4 MassTrace, Woburn, Massachusetts 01801; 2 Unit of Protein Metabolism, Department of Biochemistry and Physiology of Nutrition, German Institute of Human Nutrition, 14558 Bergholz-Rehbrücke, Germany; and 3 Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB, Scotland, United Kingdom

We have investigated whether there is a net contribution of lysine synthesized de novo by the gastrointestinal microflora to lysine homeostasis in six adults. On two separate occasions an adequate diet was given for a total of 11 days, and a 24-h (12-h fast, 12-h fed) tracer protocol was performed on the last day, in which lysine turnover, oxidation, and splanchnic uptake were measured on the basis of intravenous and oral administration of L-[1-13C]lysine and L-[6,6-2H2]lysine, respectively. [15N2]urea or 15NH4Cl was ingested daily over the last 6 days to label microbial protein. In addition, seven ileostomates were studied with 15NH4Cl. [15N]lysine enrichment in fecal and ileal microbial protein, as precursor for microbial lysine absorption, and in plasma free lysine was measured by gas chromatography-combustion-isotope ratio mass spectrometry. Differences in plasma [13C]- and [2H2]lysine enrichments during the 12-h fed period were observed between the two 15N tracer studies, although the reason is unclear, and possibly unrelated to the tracer form per se. In the normal adults, after 15NH4Cl and [15N2]urea intake, respectively, lysine derived from fecal microbial protein accounted for 5 and 9% of the appearance rate of plasma lysine. With ileal microbial lysine enrichment, the contribution of microbial lysine to plasma lysine appearance was 44%. This amounts to a gross microbial lysine contribution to whole body plasma lysine turnover of between 11 and 130 mg · kg-1 · day-1, depending on the [15N]lysine precursor used. However, insofar as microbial amino acid synthesis is accompanied by microbial breakdown of endogenous amino acids or their oxidation by intestinal tissues, this may not reflect a net increase in lysine absorption. Thus we cannot reliably estimate the quantitative contribution of microbial lysine to host lysine homeostasis with the present paradigm. However, the results confirm the significant presence of lysine of microbial origin in the plasma free lysine pool.

lysine kinetics; amino acid requirement; tracer balance; lysine-15-nitrogen; gas chromatography-combustion-isotope ratio mass spectrometry


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