Metabolomics

Metabolomics is the "systematic study of the unique chemical fingerprints that specific cellular processes leave behind" - specifically, the study of their small-molecule metabolite profiles.^[1] The metabolome represents the collection of all metabolites in a biological organism, which are the end products of its gene expression. Thus, while mRNA gene expression data and proteomic analyses do not tell the whole story of what might be happening in a cell, metabolic profiling can give an instantaneous snapshot of the physiology of that cell. One of the challenges of systems biology is to integrate proteomic, transcriptomic, and metabolomic information to give a more complete picture of living organisms.

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Metabolome

Metabolome refers to the complete set of small-molecule metabolites (such as metabolic intermediates, hormones and other signalling molecules, and secondary metabolites) to be found within a biological sample, such as a single organism.^[2] The word was coined in analogy with transcriptomics and proteomics; like the transcriptome and the proteome, the metabolome is dynamic, changing from second to second. Although the metabolome can be defined readily enough, it is not currently possible to analyse the entire range of metabolites by a single analytical method. In January 2007, scientists at the University of Alberta and the University of Calgary completed the first draft of the human metabolome. They have catalogued and characterized 2500 metabolites, 1200 drugs and 3500 food components that can be found in the human body.^[3] Metabolomics in today's world carries on its shoulders the huge responsibility of providing a detailed description of human pathways and their workings.

Metabolites

Metabolites are the intermediates and products of metabolism. The term metabolite is usually restricted to small molecules. A primary metabolite is directly involved in the normal growth, development, and reproduction. A secondary metabolite is not directly involved in those processes, but usually has important ecological function. Examples include antibiotics and pigments.

The metabolome forms a large network of metabolic reactions, where outputs from one enzymatic chemical reaction are inputs to other chemical reactions. Such systems have been described as hypercycles.

Metabonomics

Metabonomics is defined as "the quantitative measurement of the dynamic multiparametric metabolic response of living systems to pathophysiological stimuli or genetic modification". This approach originated in Imperial College London and has been used in toxicology, disease diagnosis and a number of other fields.^[4]

There is some disagreement over the exact differences between 'metabolomics' and 'metabonomics'; in general, the term 'metabolomics' is more commonly used. The difference between the two terms is not related to choice of analytical platform: although metabonomics is more associated with NMR spectroscopy and metabolomics with mass spectrometry-based techniques, this is simply because of usages amongst different groups that have popularized the different terms. While there is still no absolute agreement, there is a growing consensus that the difference resides in the fact that 'metabolomics' places a greater emphasis on comprehensive metabolic profiling, while 'metabonomics' is used to describe multiple (but not necessarily comprehensive) metabolic changes caused by a biological perturbation. In practice, there is still a large degree of overlap in the way the terms are used, and they are often in effect synonymous.

History

Metabolic biochemists have arguably been 'doing metabolomics' for decades. The chromatographic separation techniques that made the initial detection of metabolites possible were developed in the late 1960's, which marks the technical origin of the field. ^[5]

Metabolomics began develop in 1970 by Arthur Robinson investigating Pauling's ideas as to whether biological variability could be explained on the basis of far wider ranges of nutritional requirements than what was generally recognized. In analyzing the "messy" chromatographic patterns of urine from vitamin B6-loaded subjects, Robinson realized that the patterns of hundreds or thousands of chemical constituents in urine contained much useful information.

Although it was not called metabolomics, the first paper devoted to this topic was titled, “Quantitative Analysis of Urine Vapor and Breath by Gas-Liquid Partition Chromatography”, by Robinson and Pauling in 1971 and published in the Proceedings of the National Academy of Sciences. Since then, Robinson has had nineteen more papers published on the quantitative patterns of metabolites in body fluids (see below). Robinson and colleagues have identified several diseases, conditions, and physiological age based on this data. It was his expectation that body fluid analysis can be optimized to make a low cost, information-rich, medically-relevant means of measuring metabolically-driven changes in functional state, even when the chemical constituents are all in the “normal range”.

The core idea that Robinson conceived is that information-rich data that reflects the functional status of a complex biological system resides in the quantitative and qualitative pattern of metabolites in body fluids. Twenty years later, others began to realize the value of this approach, and interest in this has mushroomed. The name metabolomics was coined in the 1990s (the first paper using the word metabolome is Oliver, S. G., Winson, M. K., Kell, D. B. & Baganz, F. (1998). Systematic functional analysis of the yeast genome. Trends Biotechnol. 16, 373-378), and in 2004 a society was formed to promote its study. Many of the bioanalytical methods used for metabolomics have been adapted (or in some cases simply adopted) from existing biochemical techniques. Two characteristics common to metabolomic research are:

1. Metabolites are profiled without bias towards a specific metabolite or group of metabolites
2. Relationships between the metabolites are characterized, currently mostly by multivariate methods.

On January 23rd, 2007, the Human Metabolome Project, led by Dr. David Wishart of the University of Alberta, Canada, completed the first draft of the human metabolome, consisting of 2500 metabolites, 1200 drugs and 3500 food components.

Analytical technologies: separation methods

There are two issues to be addressed for metabolite analysis: 1. separation of the analytes, usually by chromatography. Electrophoresis, particularly capillary electrophoresis, is also used. 2. Detection of the analytes, following separation by chromatographic or other methods.

• Gas chromatography, especially when interfaced with mass spectrometry (GC-MS), is one of the most widely used and powerful methods. It offers very high chromatographic resolution, but requires chemical derivatization for many biomolecules: only volatile chemicals can be analysed without derivatization. (Some modern instruments allow '2D' chromatography, using a short polar column after the main analytical column, which increases the resolution still further.) Some large and polar metabolites cannot be analysed by GC.

• High performance liquid chromatography (HPLC). Compared to GC, HPLC has lower chromatographic resolution, but it does have the advantage that a much wider range of analytes can potentially be measured.

• Capillary electrophoresis (CE). So far, there are only a relatively small number of publications on use of CE for metabolite profiling. This will no doubt change, as there are a number of advantages of CE: it has a higher theoretical separation efficiency than HPLC, and is suitable for use with a wider range of metabolite classes than is GC. As for all electrophoretic techniques, it is most appropriate for charged analytes.

Analytical technologies: detection methods

• Mass spectrometry (MS) is used to identify and to quantify metabolites after separation by GC, HPLC, or CE. GC-MS is the most 'natural' combination of the three, and was the first to be developed. In addition, mass spectral fingerprint libraries exist or can be developed that allow identification of a metabolite according to its fragmentation pattern. MS is both sensitive (although, particularly for HPLC-MS, sensitivity is more of an issue as it is affected by the charge on the metabolite, and can be subject to ion suppression artifacts) and can be very specific. There are also a number of studies which use MS as a stand-alone technology: the sample is infused directly into the mass spectrometer with no prior separation, and the MS serves to both separate and to detect metabolites.

• Nuclear magnetic resonance (NMR) spectroscopy. NMR is the only detection technique which does not rely on separation of the analytes, and the sample can thus be recovered for further analyses. All kinds of small molecule metabolites can be measured simultaneously - in this sense, NMR is close to being a universal detector. Practically, however, it is relatively insensitive compared to mass spectrometry-based techniques; additionally, NMR spectra can be very difficult to interpret for complex mixtures.

• Other techniques. MS and NMR are by far the two leading technologies for metabolomics. Other methods of detection that have been used include electrochemical detection (coupled to HPLC) and radiolabel (when combined with thin-layer chromatography).

Key applications

• Toxicity assessment/toxicology. Metabolic profiling (especially of urine or blood plasma samples) can be used to detect the physiological changes caused by toxic insult of a chemical (or mixture of chemicals). In many cases, the observed changes can be related to specific syndromes, e.g. a specific lesion in liver or kidney. This is of particular relevance to pharmaceutical companies wanting to test the toxicity of potential drug candidates: if a compound can be eliminated before it reaches clinical trials on the grounds of adverse toxicity, it saves the enormous expense of the trials.

• Functional genomics. Metabolomics can be an excellent tool for determining the phenotype caused by a genetic manipulation, such as gene deletion or insertion. Sometimes this can be a sufficient goal in itself -- for instance, to detect any phenotypic changes in a genetically-modified plant intended for human or animal consumption. More exciting is the prospect of predicting the function of unknown genes by comparison with the metabolic perturbations caused by deletion/insertion of known genes. Such advances are most likely to come from model organisms such as Saccharomyces cerevisiae and Arabidopsis thaliana.

• Nutrigenomics is a generalised term which links genomics, transcriptomics, proteomics and metabolomics to human nutrition. In general a metabolome in a given body fluid is influenced by endogenous factors such as age, sex, body composition and genetics as well as underlying pathologies. The large bowel microflora are also a very significant potential confounder of metabolic profiles and could be classified as either an endogenous or exogenous factor. The main exogenous factors are diet and drugs. Diet can then be broken down to nutrients and non- nutrients. Metabolomics is one means to determine a biological endpoint, or metabolic fingerprint, which reflects the balance of all these forces on an individual's metabolism.^[6]

See also

• List of omics topics in biology
• Proteomics
• Cytomics
• Tumor metabolome
• Nuclear magnetic resonance
• Mass spectrometry
• 2D gel
• Protein sequencing
• PIR
• Swissprot
• Pfam
• Bioinformatics
• Glycomics
• Lipidomics
• DNA microarrays
• Systems biology

References