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Updated: Sep 30, 2026
Natural Product Discovery with LC-MS/MS Diagnostic Fragmentation Filtering: Application for Microcystin Analysis
Published on: May 31, 2019
Comparison of protein phosphatase inhibitory activity and apparent toxicity of microcystins and related compounds
Emiko Ito1, Akira Takai, Fumio Kondo
1Research Center for Pathogenic Fungi and Microbial Toxicoses, Chiba University, 1-8-1, Inohana, Chuo-ku, Japan. emiko@myco.pf.chiba-u.ac.jp
Abstract:
Two metabolites of microcystin-LR glutathione conjugate and, microcystin-cysteine conjugate, as well as microcystin-RR (MCRR) are less toxic than microcystin-LR (MCLR). In the present study, we investigated why these compounds are weakly toxic in comparison with MCLR, as the reason is still unknown and no systematic study has so far been carried out for a clarification of this issue. Although they showed almost the same inhibitory activity against protein phosphatases 1 and 2A as MCLR in vitro, the apparent toxicity of these three compounds by intratracheal administration to mice decreased to about 1/12 the level of MCLR at 100microg/kg. An immunostaining study showed that these conjugates at a sublethal dose of 200microg/kg were prominently observed in the intestine and kidney, whereas effective accumulation and bleeding were not found in the liver in spite of the larger dosage. As an explanation for these results, there may be two possibilities. First, the transport system to the liver might not function well, and second, transported toxins may be effectively eliminated by an appropriate system such as the GS-X (ATP-dependent glutathione S-conjugate exported) pump. It was concluded that the inhibitory activity against protein phosphatases is not always related to the apparent LD(50) level, and that the appearance of toxicity by microcystins depends on the balance between accumulation and metabolism in the liver.
Insights
Microcystin-RR (MCRR) and its conjugates are less toxic than microcystin-LR (MCLR) because they accumulate less in the liver. This suggests toxicity depends on liver accumulation and metabolism, not just enzyme inhibition.
Area of Science:
- Environmental Toxicology
- Biochemistry
- Pharmacology
Background:
- Microcystin-LR (MCLR) is a potent cyanotoxin.
- Metabolites like microcystin-RR (MCRR) and conjugates show lower toxicity than MCLR.
- The mechanisms behind this reduced toxicity are not well understood.
Purpose of the Study:
- To investigate the reasons for the lower toxicity of MCRR and microcystin conjugates compared to MCLR.
- To determine if in vitro protein phosphatase inhibition correlates with in vivo toxicity.
- To elucidate the role of liver accumulation and metabolism in microcystin toxicity.
Main Methods:
- In vitro assays measuring inhibition of protein phosphatases 1 and 2A.
- Intratracheal administration of toxins to mice to assess acute toxicity (LD50).
- Immunostaining studies to track toxin distribution in mouse organs (liver, intestine, kidney).
Main Results:
- MCLR, MCRR, and conjugates exhibited similar in vitro protein phosphatase inhibition.
- In vivo, MCRR and conjugates showed approximately 1/12 the toxicity of MCLR.
- Toxin accumulation was prominent in the intestine and kidney, with minimal liver accumulation and no liver damage observed.
Conclusions:
- Protein phosphatase inhibition does not directly predict in vivo toxicity of microcystins.
- Reduced liver accumulation and potential enhanced elimination via systems like the GS-X pump contribute to lower toxicity.
- The overall toxicity of microcystins is determined by the balance between liver accumulation and metabolic detoxification.
