Exploring the toxicity of PbTx-2 from Karenia brevis: Insights into human thioredoxin system inhibition and modulation by brevenal

Eman A. Taher, Yuan Liu, Kathleen Rein

Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, USA;
The Water School, Department of Marine and Earth Science and Department of Chemistry and Physics, Florida Gulf Coast University, Fort Myers, FL 33965, USA.

The Florida red tide dinoflagellate, Karenia brevis produces a group of neurotoxins known as brevetoxins (PbTxs). PbTxs can cause a variety of neurological symptoms in humans who consume PbTx-contaminated shellfish and respiratory distress through aerosol exposure notably in asthmatics. It has also been found that PbTxs induce oxidative stress in treated cells or animals exposed to red tide. Our group has further demonstrated that PbTx-2, the most abundant of the PbTxs, can inhibit the thioredoxin-thioredoxin reductase system (Trx/TrxR system). Trx/TrxR system is one of the major antioxidant systems that combat oxidative stress in cells. However, it remains unknown how PbTx2 exhibits inhibitory effects on the human Trx/TrxR. We hypothesize that PbTx-2 inhibits hTrxR-1 through the formation of Michael adduct on the C-terminal Sec residue. To investigate our hypothesis, we assessed the impact of PbTx-2 on the activity of both hTrxR-1 and its Sec residue-deficient mutant. Our findings revealed that PbTx-2 exerted an inhibitory effect on hTrxR-1 activity across the tested substrates. These results suggest a specific targeting of Sec by the toxin, as evidenced by the distinct response observed in the mutant. Efforts to counteract PbTx-2's inhibitory activity involve competitive assays with naturally occurring compounds. Notably, our data reveal that K. brevis also produces a non-toxic compound (brevenal). Brevenal not only inhibits the PbTx-2 's effect on hTrxR-1  but enhances its activity. These findings offer valuable insights into the mechanism of PbTx-2 action on hTrxR-1, which may contribute to the prevention and treatment of PbTx-2-related oxidative stress.