The metabolomics of acute alcohol abuse
Mason, Shayne William
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Alcohol is a substance used and abused by many individuals. The metabolic perturbations caused by excessive alcohol consumption are widespread throughout the human body. One of the primary consequences of alcohol abuse, particularly acute alcohol abuse, is very high levels of NADH formed from excessive ethanol oxidation. A high NADH:NAD+ ratio shifts the redox potential of the cells, shifting the normal physiological equilibrium, particularly within NAD-dependent dehydrogenase-catalyzed reactions. These particular reactions occur within various metabolic pathways, such as: citric acid cycle, glycolysis and branched-chain amino acid catabolism. As such, a disruptive effect within these metabolic pathways results in the slight accumulation of perturbation markers that can be associated with alcohol abuse. Isolation and identification of these widespread perturbation markers is difficult as they only occur in quantities only slightly higher than normal physiological values. Metabolomics makes for a very aptly used technique as it takes a holistic approach, taking into consideration the entire metabolic profile; and, with the aid of bioinformatics, is able to isolate and identify particular variables/metabolites of interest and accredit them as the variables responsible for the greatest variation between control and experimental groups. A novel approach used within this investigation effectively reduced the voluminous metabolomics data generated allowing for more efficient multivariate analysis. Application of three separate statistical models, namely: (1) Unfolding PCA, (2) Cross-sectional PCA, and (3) ANOVA Simultaneous Component Analysis (ASCA), were used for analyzing the complex 3-dimensional data set created within this acute alcohol abuse investigation. Each model presented certain strengths and difficulties. Taking into consideration the results from all 3 models, the first phase of this investigation confidently illustrates the differentiation between control cases and individuals administered an acute alcohol dose and, subsequently allow for variables responsible for this separation to be: identified as variables of importance, selected and categorized into specific pathways and, finally, labelled as perturbation markers. Through experimental observation it was noted that a large number of perturbation markers associated with the branched-chain amino acid pathway were present within the experimental cases. A hypothesis was created from this observation, re-enforcing the principle that metabolomics is a hypothesis-generating system. The subsequent second phase of this investigation involves a targeted experimental protocol aimed at evaluating the proposed hypothesis, with a focus on three secondary metabolites of the isoleucine catabolism pathway (ethylhydracrylic acid, tiglylglycine and 2-methyl-3hydroxybutyric acid). Results of this targeted approach show a definite perturbance, similar to a very minor inherited metabolic disorder, occurs within the isoleucine catabolism pathway in response to an acute alcohol dose. As to our knowledge, no information pertaining to the influences of acute alcohol abuse (or even chronic alcohol abuse) within the branched-chain amino acid pathway exists within the current literature, as of date. As such, the experimental observations presented and evaluated within this investigation provide a novel and more in-depth insight into the ethanol-induced perturbances within human metabolism.
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