The altered fatty acylcarnitines, amino acids and organic acids detected in tuberculosis patient urine
Abstract
Mycobacterium tuberculosis is estimated to infect approximately one-third of the world’s
population, which can lead to an active, symptomatic disease called tuberculosis (TB), or to
asymptomatic states, often referred to as latent TB infection. In 2015 alone, 10.4 million new
TB cases were reported, resulting in an estimated 1.8 million deaths. Since the discovery of
M. tuberculosis in 1882 by Robert Koch, a vast amount of genomics, proteomics and
transcriptomics data have been generated, leading to our current understanding of
M. tuberculosis and TB. Most of the data generated from studies used M. tuberculosis
cultures; however, it is well-known that this organism’s metabolism and growth in culture
differs greatly from growth in the human host, where many different growth mechanisms and
energy substrates are preferentially used. Furthermore, very little research to date has
focused on the adaptations of M. tuberculosis to the host’s defence mechanisms or growth
environment, or for that matter, the host’s adaptations or altered metabolic state in response
to the infectious pathogen. This is important since the pathophysiology of M. tuberculosis is
directly linked to its metabolism and complex physiology, and to that of the host. Additionally,
this pathogen can utilise numerous growth substrates, either by scavenging this from the
host or via de novo biosynthesis, in order to ensure its own survival.
Metabolomics has served well to expand the current knowledge of the disease and has
contributed towards the improved diagnosis and treatment thereof, due to its unique capacity
for identifying new disease biomarkers. Metabolomics is defined as the unbiased
identification and quantification of the entire metabolome in a specific biological system, with
the use of highly advanced analytical instruments, together with various statistical,
computational and mathematical analyses. Metabolomics has enabled the identification of
new metabolite markers in sputum, blood and urine from TB patients, describing novel
M. tuberculosis metabolic pathways and host adaptations. Apart from their possible
diagnostic applications, many of these new TB metabolite markers have contributed to the
existing knowledge of the biology of the causative pathogen, including various underlying
disease mechanisms related to M. tuberculosis drug resistance and virulence, as well as the
mechanisms of TB drug action and related side-effects in the host. To date, however, very
little data has been published on urine from TB patients, which can be considered an ideal
sample matrix to identify markers associated with this host–pathogen interaction.
Considering this, in this investigation, a combined semi-targeted liquid and gas
chromatography mass spectrometry metabolomics approach was used to compare the
urinary fatty acylcarnitines, amino acids and selected organic acids of active TB patients with
that of healthy individuals, in order to better characterise the TB-induced alterations to the host metabolome. The generally elevated concentrations of the fatty acylcarnitines and
amino acids are most likely due to TB-cachexia. However, the significantly elevated
concentrations of arginosuccinate, asparate (and associated asparagine), ornithine (and
associated proline and hydroxyproline) and glutamate (and associated glutamine) in
particular, indicate a urea cycle abnormality, due to inhibition of N-acetylglutamate synthase
by the accumulating propionyl-CoA, isovaleryl-CoA and methylmalonyl-CoA in TB patients.
Furthermore, elevated propionylcarnitine, methylmalonate and methylcitrate in the TB patient
urine are associated with a vitamin B12 deficiency, which deserves further investigation.
Lastly, various metabolites indicative of lactic acidosis, ketoacidosis, oxidative stress and
liver damage were identified in the urine of the TB patients