Estimation of the microcystin content in cyanobacterial field samples from German lakes using the colorimetric protein–phosphatase inhibition assay and RP‐HPLC

The protein–phosphatase (PP‐1) inhibition potency of microcystin was employed to estimate the microcystin content in cyanobacterial field samples from three German eutrophic lakes collected mainly in summer 1996. The microcystin content of lyophilized samples extracted with aqueous methanol was determined on the basis of the colorimetric PP‐1 inhibition assay with p ‐nitrophenyl phosphate as substrate. With the test system described, an IC 50 of 1 ng/mL reaction volume (∼0.6 nM PP‐1) was determined for microcystin‐LR standard, corresponding to 20 ng microcystin‐LR per milliliter sample volume. The microcystin content of field samples estimated on the basis of the PP‐1 inhibition assay was compared to that obtained by reversed phase high performance liquid chromatography (RP‐HPLC), and a good correlation was observed ( r =0.9202, P <0.0001). The mean difference in microcystin content based on PP‐1 inhibition was determined to be 27.9±22.9% ( n =50) related to measurements based on RP‐HPLC analysis. There was no observation of false positive or false negative results in this study. The results confirm the suitability of the PP‐1 inhibition assay with regard to a preliminary screening of cyanobacterial field samples for microcystin. ©1999 John Wiley & Sons, Inc. Environ Toxicol 14: 23–29, 1999