The essential amino acid tryptophan is an important substrate for the synthesis of
serotonin, melatonin, tryptamine, proteins and the kynurenines. The aim of this study was
to investigate tryptophan metabolism along the kynurenine pathway in a low income sub-
Saharan HIV/AIDS patient population from the Gauteng Province of South Africa.
The first objective was to develop and validate a novel gas chromatography mass
spectrometry method to enable reliable quantification of tryptophan and metabolites of the
kynurenine pathway in plasma. Validation parameters for the detection of tryptophan,
kynurenine, quinolinic acid and nicotinamide conformed to international criteria for newly
developed methods. The next objective of the study was to find an appropriate biomarker
against which to express the results. Several substances previously described as indicators
were assessed and compared, including plasma neopterin, procalcitonin, C-reactive protein,
the cytokines IL-2, IL-4, IL-6, IL-10, TNF, and IFN-gamma, as well as factors routinely
measured and elsewhere described as biomarkers in HIV, i.e., albumin, the albumin/globulin
ratio, haemoglobin and red cell distribution width. Neopterin was shown to be superior as
indicator of pro-inflammatory status, as indicator of the degree of immune deficiency, to
predict disease progression, to distinguish between patients with and without tuberculosis
co-infection and to reflect the success of highly active antiretroviral treatment (HAART).
In the analyses of the kynurenine pathway metabolites, tryptophan levels were seen to be
significantly lower (24.36 ± 4.14 vs. 43.57 ± 11.85 μmol/l; p<0.0001), while the activity of the
enzyme, indoleamine 2,3 dioxygenase (IDO), (K/T:136.03 vs. 52.18; p<0.001), as well as
kynurenine (3.21 ± 1.33 vs. 2.14 ± 0.45 μmol/l; p<0.001) and quinolinic acid (4.46 ± 2.32 vs.
0.25 ± 0.058 μmol/l; p<0.001) levels were significantly higher in the total patient group
(n=105) than in the control group (n=60). Patients on HAART showed not only significantly higher CD4 counts (296.21 ± 195.50 vs. 170.05 ± 167.26 cells/μl; p=0.003), but also lower
inflammatory activity (neopterin: 35.51 ± 35.70 vs. 66.63 ± 40.73 nmol/l; p<0.001 and IL-6:
9.56 ± 12.54 vs. 15.04 ± 19.34 pg/ml; p<0.05), lower IFN-γ (41.43 ± 14.14 vs. 53.68±34.39
pg/ml; p<0.05), higher tryptophan levels (25.13 ± 3.80 vs. 22.04 ± 4.32 μmol/l; p=0.033),
lower kynurenine levels (3.08 ± 1.28 vs. 3.58 ± 1.42 μmol/l; p=0.144) and lower quinolinic
acid levels (4.03 ± 2.04 vs. 5.77 ± 2.65μmol/l; p=0.072) than patients not on HAART.
Tryptophan depletion and IDO activity, as well as the levels of kynurenine and quinolinic
acid, were generally greater than in populations from developed countries. Indications are
that this can be ascribed to higher levels of inflammatory activity at comparable levels of
immune deficiency in the disadvantaged population of this study. The degree of tryptophan
depletion and quinolinic acid accumulation found could negatively impact on the physical
and neuropsychiatric wellness of the population. Correlations between quinolinic acid, and
nicotinamide levels showed a significant contribution of kynurenine pathway metabolism to
the plasma levels of nicotinamide. This de novo synthesis of nicotinamide could offer
protection against niacin deficiency and NAD depletion in populations with inadequate
dietary intake. This is the first study to assess plasma tryptophan, kynurenine, quinolinic
acid and nicotinamide levels, as well as IDO activity, pro-inflammatory status and IFN-γ
levels, simultaneously in one population and to compare it to that of HIV/AIDS patients in
developed countries. / Thesis (PhD)--University of Pretoria, 2015. / tm2015 / Physiology / PhD / Unrestricted
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:up/oai:repository.up.ac.za:2263/46053 |
| Date | January 2015 |
| Creators | Bipath, Priyesh |
| Contributors | Viljoen, Margaretha |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | © 2015 University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. |
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