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Metals are potentially good biomarkers because they cannot be created or destroyed the way organic analytes can. They are frequently an integral part of many biological systems, such as protein binding. One property of metals is their ability to act as a label for small-molecule metabolism. Specifically, many small organometallic molecules can be measured in a biological compartment using the metal as the label.
Mancozeb (MZ), a fungicide comprised of an organic backbone and a metal moiety in a 9:1 ratio of Mn to Zn, is neurotoxic. Its metabolite, ethylene thiourea (ETU), is a known teratogen, carcinogen, and anti-thyroid compound. It is one molecule that has both an organic moiety and a metal analyte. Measuring the organic backbone is often more difficult than measuring the metal because the organic moiety appears to be unstable. Measurement of MZ in biological matrixes and demonstration of its uptake by neuronal cells proved difficult by conventional LC/MS.
Quantification by liquid chromatography/mass spectrometry of the organic analyte inside the neuronal cells showed that about 8% of the compound crossed the cell membrane. This was significant because no one had measured MZ inside the cell, even though its neurodegenerative properties were known. The results were not completely conclusive because the precursor ion could not be measured inside the cell, only product ions. ICP/MS was used to measure the Mn content inside the cell and discover that about 8% of the MZ had entered the cell. The agreement between the two methods was significant and demonstrated the utility of the metal as a confirmatory analysis. It also confirmed that the Mn had entered the cell and not just the organic backbone. This presentation will focus on the use of metals as labels in biological systems, including analytical methods and stable isotope labeling.