Determination of cyanobacterial hepatotoxins directly in water using a protein phosphatase inhibition assay

A colorimetric phosphatase inhibition assay using protein phosphatase 2A and p-nitrophenyl phosphate as substrate for determining cyanobacterial peptide hepatotoxins directly in water without sample preconcentration has been developed. The assay uses commercially available materials and is much more simple to use than similar procedures using radiolabelled substrates. It has similar sensitivity to the radiolabelled assays and, with a working range of around 0.2–1 μg/L, is able to determine these toxins at concentrations below the provisional World Health Organisation guideline of 1 μg/L for microcystin-LR. The method appears robust and not to be affected by the sample matrix apart from possibly some components of cellular material if present at very high levels in extracts of cyanobacterial material. It is not affected by the presence of low levels of methanol in sample extracts.