Nitrogen isotopic discrimination in dietary amino acids: The threonine anomaly.
BACKGROUND
The "Threonine Anomaly" relates to an observation made 25 years ago on the change in Thr nitrogen isotopic ratio in mammalian metabolism. Unlike all other amino acids, Thr in body protein is found to be depleted (rather than enriched) in15N relative to dietary Thr. Interpreting isotopic discrimination has become a useful source of ecological and palaeodietary information and it is desirable that the underlying processes are understood.


METHODS
The principal enzyme of threonine catabolism, suggested to be responsible for the anomaly, threonine dehydratase, was prepared from rat liver. A time course of incubation of the enzyme with pure threonine was followed, and samples of residual threonine prepared for isotopic analysis by combustion in an automated carbon and nitrogen analyser coupled to a continuous flow isotope ratio mass spectrometer.


RESULTS
We show experimentally, in vitro, that the enzymic reaction catabolising Thr cannot be responsible for its15N depletion. Plots of delta15N against both reaction time course and percentage completion show in fact an accelerating enrichment.


CONCLUSIONS
A previously advanced suggestion that the unique catabolic mechanism for threonine was responsible for the anomalous depletion in15N is clearly not the case. We therefore offer alternative explanations, based on threonine's role at an organismal rather than cellular level. Copyright © 2016 John Wiley & Sons, Ltd.
Department of Biochemistry, Dalhousie University, 5850 College Street, Halifax, NS, B3H 4R2, Canada. cwallace@dal.ca.
Carmichael J A
http://orcid.org/0000-0002-3546-1558
CJ
Wallace
University of Oxford, Research Laboratory for Archaeology, South Parks Road, Oxford, OX1 3QJ, UK.
Robert E M
RE
Hedges
D000818:Animals; D004032:Diet; D008099:Liver / Q000201:enzymology; D013058:Mass Spectrometry; D009587:Nitrogen Isotopes / Q000032:analysis* / Q000737:chemistry / Q000378:metabolism*; D051381:Rats; D013912:Threonine / Q000032:analysis* / Q000737:chemistry / Q000378:metabolism*; D013913:Threonine Dehydratase / Q000378:metabolism
D009587:Nitrogen Isotopes; D013912:Threonine; D013913:Threonine Dehydratase