Abstract
Permanganate pretreatment of drinking water is effective in transforming dissolved, noxious contaminants and in reducing halogenated by-products. Permanganate targets specific compounds such as taste and odor compounds, disinfection precursors, manganese, and natural organic contaminants that are not removed readily by conventional treatment alone. Cyanobacterial blooms (cHABs) can increase disinfection by-product precursors as well as the cyanotoxin, microcystin (MC), a potent liver toxin. MC toxicity is conferred by a unique, conserved amino acid, Adda, that inhibits protein phosphatase 1 and 2A. Although over 150 MC congeners have been reported, thousands of MCs are statistically possible. Over the last ten years, one congener, MC-LA, has been reported with increasing frequency, making it one of the most common cyanotoxins identified in North American freshwater systems; yet its oxidation has not been widely studied. Frequently, Adda specific enzyme-linked immunosorbent assay (ELISA) and protein phosphatase inhibition assay (PPIA) are used to quantitate total MCs to evaluate treatment efficiency and exposure. Anecdotal reports suggest that MC degradation products can cause interference with the Adda-ELISA. MC-LA was used as the model MC compound in this study. PPIA quantitation of MC-LA in water agreed with liquid chromatography high resolution mass spectrometry (LC/HRMS), whereas the ELISA quantitation did not agree with LC/HRMS quantitation. We determined the second order rate constant for MC-LA as 118 ± 9 M−1 s−1, the activation energy to be 21.2 kJ mol−1, and the rate to be independent of pH between pH 6 and 9. Ten oxidation products (OPs) were observed by LC/HRMS and three primary reaction pathways are proposed. The reaction pathways were used to explain differences in the quantification by Adda-ELISA, HRMS, and PPIA. The oxohydroxylation of MC-LA produced two major OPs, C46H67N7O14 [M+H] + = 942.4819 and C46H69N7O15 [M+H]+ =960.4925. Major OPs may contain an unmodified Adda and are the likely cause of interference with the Adda-ELISA. Several governmental agencies recommend the use of the Adda-ELISA to determine the MC quantitation for treatment efficiency and customer exposure; yet our results suggest that these or other OPs interfere with the Adda-ELISA causing artificially high values for total MCs.
Highlights
The severity and frequency of cyanobacteria blooms are increasing due to a combination of factors including increased nitrogen and phosphorous pollution, climate change, and altered system ecology which in some cases may be the result of invasive species [1, 2]
This program uses published kinetics, pH dependent kinetics, and activation energies in their predictive empirical model. The authors of this tool have identified four reoccurring problems: (1) only a limited number of MCs have been investigated; (2) often temperature and pH dependent experiments are not performed; (3) the impact of NOM and cyanobacteria cells on oxidation kinetics is highly variable; and (4) kinetics determined by LC/MS are different from kinetics determined by Adda-enzyme-linked immunosorbent assay (ELISA)
Concentrations, we are able to show that oxidation products (OPs) were likely binding to the Adda-ELISA antibody
Summary
The severity and frequency of cyanobacteria blooms are increasing due to a combination of factors including increased nitrogen and phosphorous pollution, climate change, and altered system ecology which in some cases may be the result of invasive species [1, 2]. Cyanobacteria blooms impact drinking water treatment efficiency by increasing chemical demand and particulate load, resulting in increased risk of pathogen and cyanotoxin breakthrough [3, 4]. Quantitative tools used to monitor MC drinking water treatment removal efficiency and exposure are critical for consumer safety. These tools, i.e., assays, would determine the concentration of MCs as well as the toxicity of the water and would be simple enough to be performed at the treatment plant. Our goal was to vet the use of two commercial MC assays to quantitate MCs
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