Science Inventory

pH dependent octanol–water partitioning coefficients of microcystin congeners

Citation:

McCord, J., J. Lang, D. Hill, M. Strynar, AND N. Chernoff. pH dependent octanol–water partitioning coefficients of microcystin congeners. JOURNAL OF WATER AND HEALTH. IWA Publishing, London, Uk, 16(3):340-345, (2018). https://doi.org/10.2166/wh.2018.257

Impact/Purpose:

Microcystins (MC) are a class of cyclic heptapeptides produced by fresh water cyanobacteria (blue-green algae), primarily those in the genus Microcystis, with worldwide occurrence (Dietrich and Hoeger 2005). Cyanobacteria are photosynthetic organisms that constitute an important component of varied aquatic ecosystems where they are important primary producers and also play key roles in nitrogen fixation. Under suitable environmental conditions, cyanobacteria populations may undergo explosive growth and toxin-producing species will form large hazardous algal blooms (HABs), which are increasing in frequency as the result of anthropogenic eutrophication and climate change (Hallegraeff 1993, O’Neil et al. 2012, Paerl et al. 2001). While multiple toxins are produced by cyanobacterial blooms, microcystins are among the most commonly reported hepatotoxic compounds found in surface water (Chorus and Bartram 1999, D’Anglada and Strong 2015, Loftin et al. 2016, Sivonen and Jones 1999). Microcystins are selectively absorbed into liver cells by Organic Anion Transport Polypeptides (OATPs) and inhibit protein phosphatases,(Campos and Vasconcelos 2010, Pereira et al. 2013) resulting in liver cell death, dysfunction, and, if sufficient exposure has occurred, lethality. (Bengis et al. 2016, Briand et al. 2003, Lone et al. 2015) The microcystin class shares a common peptide structure containing multiple non-standard amino acids and two variable amino acid moieties (Fig 1), resulting in dozens of distinct forms with different physiochemical properties. Toxicity is known to vary greatly between congeners as a result of their pH dependent solubility and bioavailability,(Gupta et al. 2003, Stoner et al. 1989) and environmental fate is closely tied to both environmental pH and microcystin hydrophobicity.(Pérez and Aga 2005, Schmidt et al. 2014)

Description:

Hazardous algal blooms can generate toxic compounds with significant health impacts for exposed communities. The ubiquitous class of algal toxins known as microcystins exhibits significant heterogeneity in its peptide structure, which has been minimally studied, given the significant impact this has on hydrophobicity, acid/base character and related environmental fate and health effects. Octanol–water partition coefficients for the microcystin congeners MCLR, MCRR, MCLY, MCLF, and MCLA were calculated over an environmentally and physiologically relevant pH range. Microcystin-LR log(Kow) partition coefficient values were found to be consistent with previously established literature values, 1.67 to −1.41 between pH 1 and 8. Microcystin RR was found to be pH insensitive with a log(Kow) of −0.7. The remaining congeners exhibit similar pH dependence as MCLR, with systematic increases in hydrophobicity driven by the introduction of more hydrophobic residues to their variable amino acid region. The variation in pH dependent hydrophobicity suggests increased propensity for bioaccumulation and alternate environmental fates for differing microcystin forms, requiring further investigation.