<i>Karenia brevis</i>, the Florida red tide dinoflagellate produces a suite of neurotoxins known as the brevetoxins. The most abundant of the brevetoxins PbTx-2, was found to inhibit the mammalian thioredoxin- system, whereas the PbTx-3 has no effect on this system. The thioredoxin system is a major cellular reductant system present in the cell, whose role is to maintain cellular redox homeostasis. It does this in part, by regulating the activity of many other enzymes controlling cellular processes. This system also provides antioxidant defense, reducing destructive reactive oxygen species (ROS). The system is comprised of thioredoxin (Trx) which reduces target protein disulfide bridges by thiol-disulfide exchange and thioredoxin reductase (TrxR) which utilizes NADPH to reduce oxidized Trx, returning it to its active form. PbTx-2 inhibits the reduction of Trx by TrxR. On the other hand, PbTx-2 activates the reduction of small disulfides such as 5, 5’-dithio-bis-(2-nitrobenzoic acid) by TrxR.  PbTx-2 has an α, β-unsaturated aldehyde moiety which functions as an efficient electrophile and selenocysteine conjugates are readily formed. It is proposed that the mechanism of inhibition of thioredoxin reduction is via the formation of a Michael adduct between selenocysteine<sup> </sup>and the α, β-unsaturated aldehyde moiety of PbTx-2. To our surprise, while PbTx-3 does not inhibit Trx reduction by TrxR, is does activate DTNB reduction. PbTx-3 does not have the α, β-unsaturated aldehyde moiety suggesting that reaction with the selenocysteine is essential for Trx reduction but not for DTNB reduction. It also suggests that small disulfides compete more effectively for TrxR reduction on the presence of brevetoxins. We have attributed the unusual activity of the brevetoxins toward TrxR to the functionality and the large size of the brevetoxin molecule. The dysregulation of TrxR activity leads to an increase in ROS and oxidative stress in cells and represents another mechanism of toxicity for these two toxin classes. Competition assays with known TrxR inhibitors have helped to establish the binding sites of these toxins.

Brevetoxin is an effector of mammalian thioredoxin reductase (TrxR)

2019 Spring National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.