Tryptophan
is an essential amino acid that plays an important role
in cell metabolism, and kynurenine is its main metabolic pathway.
By using ultra-high-performance liquid chromatography coupled to electrospray
ionization triple–quadrupole mass spectrometry, tryptophan
and kynurenine were determined using amlodipine as an internal standard.
The analysis was carried out on an ACE-C18 (4.6 mm × 50 mm, 5
μm) reversed-phase analytical column using the gradient elution
mode. For quantitative determination, amlodipine was used as an internal
standard. Detection was performed using multiple reaction monitoring
in electrospray ionization mode at m/z 205.1 → 117.7 and 187.9 for tryptophan, m/z 209.1 → 146 and 93.9 for kynurenine, and m/z 409.2 → 294.1 for the internal
standard. Good linearity of the analyte to internal standard peak
area ratios was seen in the concentration range 1.25–4000 ng/mL
for tryptophan and 0.5–1600 ng/mL for kynurenine. The method
showed excellent linearity with regression coefficients of 0.99 for
kynurenine and 0.996 for tryptophan. The limits of quantification
were 0.55 ng/mL for tryptophan and 0.47 ng/mL for kynurenine. The
% RSD for all analytes ranged from 0.3 to 3.4% for intraday and 0.4
to 8.9% for interday experiments. A simple LC–MS/MS method
has been developed and validated for measuring Kyn and Trp by using
an affordable and more easily available internal standard, which is
amlodipine.
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