An antidote to one of the most potent marine venoms has been found in frogs

Saxitoxin is produced by certain microalgae and cyanobacteria. The toxin accumulates in shellfish and can enter the human body through food. It disrupts the transmission of signals to nerves and muscles, meaning that severe poisoning can lead to paralysis and respiratory arrest. There is currently no specific antidote for it.

However, it is too early to speak of a ready-to-use medicine. Saxifilin has only been tested on mice, and its safety and efficacy in humans have yet to be verified. The study has been published in the journal ⁠Nature Communications.

Details

Researchers at the University of California, San Francisco, studied the protein saxifylin. It is found in the American bullfrog and certain other frogs and toads that are resistant to saxitoxin.

Saxitoxin acts rapidly. Its molecules block sodium channels — a kind of ‘gate’ on the surface of nerve and muscle cells. Without these channels, cells cannot transmit electrical signals normally. As a result, a person may experience numbness, weakness, difficulty swallowing and impaired coordination, and in cases of severe poisoning, paralysis of the respiratory muscles.

Saxifiline works in a simpler way: it binds tightly to the toxin molecules in the blood and prevents them from reaching the cells. The authors compare it to a molecular sponge that absorbs the toxin and renders it inactive.

First, the scientists tested the protein on cells containing three types of human sodium channels. These are involved in the functioning of skeletal muscles, the heart and the peripheral nervous system. Saxifiline protected all three types of channels from the toxin’s effects.

The protein was then tested on mice. The researchers carried out three different experiments:

• saxifilins was administered at the same time as the toxin;
• the protein was administered before exposure to the toxin;
• the animals were first given a lethal dose of saxitoxin, and only then the potential antidote.

The protein protected the mice in all three cases. The last experiment proved to be the most significant: saxifilin saved the majority of the animals, even though the toxin had already begun to take effect. It is precisely this sequence of events that most closely resembles a real-life poisoning scenario, where a person seeks help after having eaten contaminated molluscs.

Saxifilin not only increased survival rates but also reduced the severity of symptoms. According to the researchers, the protein itself did not cause any noticeable side effects in the mice.

To confirm the mechanism of action, the scientists also tested a modified version of saxifilin, which binds the toxin approximately 2,000 times less effectively. It did not protect the animals. This confirmed that the protein saves the organism precisely because of its ability to bind toxin molecules.

Why this is important

Saxitoxin is considered one of the most dangerous natural neurotoxins. It belongs to a family of more than 50 similar substances that can accumulate in mussels, oysters, scallops and other shellfish.

Poisoning caused by such toxins is known as paralytic shellfish poisoning. There is no specific antidote: doctors can only support breathing and other vital functions until the body has eliminated the toxin.

This new study has demonstrated for the first time in a living organism that a single natural protein is sufficient to protect against the lethal effects of saxitoxin. It is particularly encouraging that it proved effective even after exposure to the toxin.

But the road to a medicine may prove to be a long one. Saxifilin is a large protein molecule with a mass of around 91 kilodaltons. Scientists still need to determine how long it remains in the blood, whether it triggers an immune response, and at what dose it can be safely administered. Furthermore, the results of experiments on mice do not guarantee that the protein will be equally effective in humans.

The authors aim to create smaller versions of saxifilin. Such molecules could potentially move more quickly through the body and bind to different variants of the toxin.

Background

A so-called ‘red tide’ is a mass proliferation of microscopic aquatic organisms, some of which produce hazardous substances. The water does not always turn red during this process, which is why scientists more often refer to it as a harmful algal bloom.

Molluscs filter the water and accumulate the toxin, but do not necessarily die themselves. As a result, the toxin can enter the bodies of humans and animals that consume them.

The history of this discovery dates back almost a hundred years. In the late 1920s and early 1930s, Hermann Sommer, a researcher at the University of California, San Francisco, discovered that ‘mussel poison’ is not produced by the molluscs themselves, but by the microorganisms on which they feed. He also observed that some frogs are resistant to poisoning.

Later, scientists identified saxifilin in frogs and discovered how it binds to saxitoxin. However, prior to this study, it was unknown whether this mechanism could save a living organism that had already been poisoned.

In the future, saxifilin may prove useful not only as the basis for a medicine. Researchers suggest that its ability to locate and bind the toxin will enable molluscs and water to be tested more quickly for dangerous contamination.

Source

The study by Samantha A. Nixon, Sandra Zakrzewska and co-authors, “Saxiphilin functions as a ‘toxin sponge’ protein that counteracts the effects of saxitoxin poisoning”, was published in the journal *Nature Communications* in 2026.