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1 Laboratory of Human
Nutrition,
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 · day1,
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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