Ricin: Structure and function

Ricin

Found inside of the seeds of the castor oil plant and in castor beans, the cytotoxin, Ricin, is known to be one of Earth's most toxic proteins. Ricin was officially discovered in the year 1888 during the investigation of castor seed toxicity, which had been a mystery to scientists since their observations of its effects in the late nineteenth century. German scientist, Peter Hermann Stillmark, was able to extract this compound from the castor seed and purify it into the structure we see today. Originally thought to be used in aiding cancer patients, its highly toxic nature to humans deemed it too dangerous to use in a medical setting. Doctors and scientists have even worked together to pick apart the protein and use each chain in cancer therapy separately. On the other hand, it’s toxicity attracted the attention of criminals, military, and terrorists who came up with the idea of using it as a weapon in bio-terroristic attacks.

Toxicity

We know Ricin to be a very toxic protein based on its interaction with mammals, but in terms of plants, it doesn’t show to have much effect. Due to the cell structure of a plant cell, it contains a cell wall that protects the cell from external threats, including Ricin. Ricin is not able to penetrate the plant cell wall and this is why we see it take on its toxic effects in animal cells instead. When it comes to ricin toxicity in mammals, each species has varying terms of sensitivity to this protein. Some species may experience lethal effects due to Ricin while others deem it a little inconvenience. The severity of Ricin will also depend on the way an individual was exposed to it. Ricin exposure is very unlikely to be accidental and you shouldn’t worry about getting Ricin Poisoning unless you are consuming pure castor beans. Most cases of Ricin Poisoning are due to the result of mishandling the protein after its extraction to make castor oil. The modes of exposure can range from inhalation to ingestion, excluding dermal absorption. After testing ran on mice, the Ricin protein was determined to be too molecularly large to penetrate the epidermis. Due to its size, it does have detrimental effects when ingested or inhaled. Inhaling this substance is one of the most lethal ways of exposing the body to Ricin, as a lethal dose is roughly only around 5 micrograms per kilogram. In terms of ingestion, the GI tract has mechanisms to degrade this protein after it is consumed. This means that it has a less severe effect than inhalation and its lethal dose comes to about 20 milligrams per kilogram. Severity may also rely on how the Ricin was consumed. Swallowing an entire castor bean results in a reduced risk of ricin poisoning as the shell of the bean is hard to break down. Swallowing crushed seeds is much more toxic than those whose shell is still intact.

Structure

The structure of Ricin consists of two polypeptide chains known as the A and B chain that carry out different functions within the protein due to their amino acid sequences. This heterodimer’s chains are joined by disulfide bonds and have fairly similar molecular weights. In order for ricin to carry out its function, the two chains need to be linked, although they do separate further into the mechanism of action. The A chain (Ricin A) is a globular protein that consists of 8 beta sheets and 8 alpha helices. It is composed of 267 amino acids and is separated into three structural domains. The first domain is made up of parallel and antiparallel beta sheets, the second contains alpha helices, and the third consists of both. These three domains carry out the function of ribosome inactivation within the Golgi apparatus in a cell, as the active site for depurination lies on the second alpha helical domain. The enzymatic activity of this protein is mainly seen on this chain in the ricin loop, where an adenine of a target cell becomes trapped by two tyrosines. This will result in the prevention of mRNA translation. The B chain (Ricin B) is a lectin that is composed of 262 amino acids and carries out the action of binding through a galactose binding terminal. This chain does not have any alpha helical or beta sheet structure but does fold into two domains. These chains are not exclusive to castor plants and they can be found in other non toxic vegetation as well; it is only when these two chains are bound together where they are able to have devastating impacts on an organism.

Mechanism of Action

When the A chain and B chain are joined, they are able to work together to carry out enzymatic activity within an organism. Because they have two completely different properties, their individual functions work together. The main goal of ricin is to inhibit protein synthesis, a process that heavily relies on organelles called ribosomes. When these ribosomes are damaged or inactivated, this halts protein synthesis within a cell. At the start of the process, the B chain’s purpose is to gain entry into a target cell so that the A chain can carry out ribosomal deactivation. As Ricin B attaches to the galactose residues on a cell membrane, the protein is able to invade the cell through the cleavage of the cell membrane, endocytosis. Now that the B chain has done its part in the protein’s function, Chain A has entry into the cytosol of the cell. After Ricin enters the cell, the disulfide bond between the A and B chains is cleaved and the A chain is released into the cell, working its way to the Golgi network. Ricin A functions to deactivate ribosomal activity through elongation. The enzymatic activity of this protein can inactivate thousands of ribosomes per minute, making it a very fast acting protein. As stated in the structure of the protein, there is a ricin loop on chain A where two tyrosine residues play an important role in the deactivation of the ribosome. Tyr80 and Tyr123 sandwich an adenine to inhibit further protein synthesis. After protein production in a cell is stopped, it is unclear whether the two chains link back together and move on to another cell or whether other cells are affected by new ricin proteins.

Ricin Poisoning: Symptoms

Now that we understand the structure and mechanism of action behind Ricin, how can we make sure we take the correct steps in identifying if you may have been exposed to this protein? The main mechanism of this protein is its ability to halt protein synthesis. Because it doesn’t damage the cell directly and instead stops it’s functioning, the symptoms of ricin poisoning can take up to a couple days to be noticed, depending on exposure and dose. When ricin is ingested, it first manifests symptoms within the gastrointestinal tract, leading to pain and inflammation. The longer an individual is left untreated, the more severe symptoms they may experience. Gastrointestinal symptoms may progress to vomiting and blood in the stool. The loss of blood and fluid due to the symptoms can lead to more detrimental impacts as the body may start to experience the effects of dehydration and hypovolemia. When the body has low blood and water levels, it can result in organ failure and if severe enough, result in death. The inhalation of ricin can be much more lethal than ingestion as the respiratory system has no action to digest the protein like the digestive system does. Early symptoms can be displayed through throat and lung irritation such as a cough. As the poisoning progresses, it creates an allergic reaction that can cause skin irritation, chest tightness, sore throat, and asthma-like breathing. Oftentimes inhalation symptoms can become severe enough to cause chronic pulmonary disease or vascular leak syndromes. Due to the early symptoms of ricin poisoning being pretty general and common symptoms associated with common and acute illnesses, doctors can run several diagnosis tests if ricin exposure is suspected. The most common test that can detect the ricin protein in an organism is enzyme-linked immunosorbent assays, also known as the ELISA test. Other methods include testing urine or checking Ricin-antibody counts. Although there are several ways to detect if a person may be experiencing Ricin Poisoning, there is no single method that is used in standard practice as of now.

Ricin Poisoning: Treatment

Ricin’s effect on an organism's body can happen so suddenly and severely that the treatment of ricin poisoning is very variable. Scientists and doctors both recommend that those who may have consistent exposure to ricin and maybe at high risk of being poisoned receive vaccinations to prevent ricin from being able to come in contact with internal cells. It is also highly recommended that worksites that produce castor oil and have high levels of ricin circulating throughout solutions go through very extensive decontamination processes. If an individual does become poisoned, it is important that their water and blood levels are being monitored and maintained as they are given anti-ricin antibodies to protect the cells. This can extend survival time and even help the organism to fight off the ricin completely.