The following is based on- Bad Fish: The Case of Tetrodotoxin

After recovering from his TTX poisoning, Dr. Marshall Westwood decided to take a vacation. An avid birder, he decided to go to Papua New Guinea with Bill Whitlatch, an ornithologist friend of his from Montana Technical Institute. Th ree days into their trip, Bill netted bird with an orange body and black wings and head for closer study. Dr Westwood was very curious and asked Bill if he could have a closer look at the bird. After handling the bird and then later wiping his mouth with his hand, Dr. Westwood noticed that his fi ngers and lips were going numb. His mind immediately fl ashed back to the disastrous trip to Indonesia and he began to panic. Luckily, the symptoms faded before they progressed into anything more serious. His friend Bill used a key to identify the animal as a pitohui. Th e pitohui are small, social songbirds that live in Papua New Guinea. They are generally about 23 centimeters long with strong legs and a powerful beak. Their encounter was the fi rst time anyone had scientifically realized the birds’ toxicity. Before releasing the bird, Dr. Westwood collected feather and tissue samples to bring back to the lab. After returning to Montana, he set out to isolate the toxic compound that he believed was being produced by the pitohui. It appeared that the active ingredient was a homobatrachotoxin. Homobatrachotoxin is a steroidal alkaloid that is similar to batrachotoxin, the toxic principle of the Central American poison arrow frog Phyllobates aurotaenia. Batrachotoxin and homobatrachotoxin are both known to act on the voltage-sensitive sodium channels in excitable tissues. You and your colleagues received a call from Dr. Westwood asking if you could help elucidate the mechanism of action of this toxic compound. One of the hypotheses is that this toxin acts similarly to TTX.

In your first experiment, you have determined that the receptor affinity for this toxin is not as great as it is for TTX. What does this mean about the value of Kd for this toxin relative to what is known for TTX?