Further into Nardoo (cont'd Australian Expedition)

                My last post Beriberiand a Rough Australian Trip discussed the Wills-Burke expedition and the death of three out of four key participants due to a thiamine deficiency.  Near the end of that post, I explained that the men were eating foods rich in thiaminase I, which was breaking down any thiamine in their bodies.  Lack of the essential vitamin thiamine led to neurological disorders and was eventually fatal to all except John King.  One place that gave the men too much thiaminase I was fresh water mussels.  The other was nardoo, which is commonly prepared by the Aborigines without incident but was cooked incorrectly by the European explorers.

                I wanted to do a brief follow up to explain the differences in their preparations and why thiaminase I was still active in the European dish but was inactive in the Aborigine version.

                Nardoo is prepared from the plants of the genus Marsilea.  It is a fern but resembles a four-leaf clover (Figure 38.2).  The fronds, which are ground up to create a type of flour, are rich with thiaminase I.

               

                  Let’s take a step back and look more closely at the protein thiaminase I.

               

                  Figure 39.1 shows you a picture of the protein.  It is an enzyme, meaning that the protein helps a chemical reaction occur (Fun with Radioactivity post).  The particular reaction it catalyzes is shown in Figure 39.2.

                  In short, thiamine and another organic molecule (several molecules fit the bill here so the specifics aren’t important) bind to thiaminase I.  The enzyme then breaks thiamine into two pieces.  A broken thiamine is then unable to do its job in the body and leads to large problems, as demonstrated by Wills, Burke, King and Gray.

                 What are ways to ensure that this enzyme can’t do its job?

One: Destroy it.  I’ve told you previously that protein structure is essential for function (Cancerous Mutational Problems post).  If it doesn’t have the correct structure then it can’t do its job.  There are several ways to destroy protein's structure, but the simplest and most familiar one is heat.    Heat that protein up (ie. cook it)!  Heat is ruining that carefully folded protein and turning into a blob of amino acids with no function.

Think about when you cook an egg.  When it is first cracked, the whites are clear with a slightly yellow tinge.  As it cooks, the whites become opaque and stark in color.  Why?  You’re destroying all the protein inside the whites.  They are falling apart and forming a big clump of unfolded proteins.  It is safe to eat an egg after it has been cooked because any proteins in there are now dead and won’t function inside your body.  It’s also helpful to your digestive system, which further breaks down the proteins into their individual amino acids, transports them to cells, and your cells then use those amino acids to make new proteins that they need (Central Dogma post).  The circle of life.

Two: Take away its substrates.  Thiaminase I requires two things to function: thiamine and another organic molecule.  Clearly, minimizing thiamine is not an option, but it is possible to keep the other organic molecules to a minimum.  

Look at the cars outside.  They require two things to get them to move: gas and keys.  You can dip that entire car in gas but if there are no keys, you car isn’t going anywhere (unless you can hotwire a car, I suppose).  Same idea with thiaminase I.

                Our explorers went the route of #1 because that is how Europeans prepared grains: grinding and cooking.  Unfortunately, let’s quickly consider the Australian bush.  It’s HOT.   The proteins in the plant are going to be used to the hot temperatures so to destroy them, they must be cooked even hotter to mess up their proteins.  According to an article by Earl and McCleary in the journal Nature, these ferns can be boiled for 15 minutes and still have functional proteins.  That’s a hearty plant.

                The Aborigines choose route #2.  The plant is mixed with water and ground into a paste.  They are very careful to keep it away from anything else that might contain organic compounds and this minimizes how many organic molecules are present (how many keys are around to start the car).  Further diluting the paste with water ensures that if any organic molecules are present then they will be few and far between (spreading keys among 1,000 people as opposed to ten people).

                To be clear, no one knew exactly what the problem was with preparing nardoo.  It’s not like they held the plant up and said “Oh yes, there is thiaminase I here!  We will need to plan accordingly.”  It was obviously much more trial and error with preparation over time.  Whatever worked (and didn’t kill anyone) was kept around.  I would imagine it’s similar to discovering wild mushrooms or even rhubarb (the flesh of rhubarb is okay but the leaves are quite poisonous.  Crazy!).

Link to Protein Folding post, referenced in Figure 39.1.

REFERENCES

Earl and McClearly. “Mystery of the poisoned expedition.” Nature (1994) 368(6473) pgs 683 – 684.

Campobasso et al. “Crystal Structure of Thiaminase I- from Bacillus thiaminolyticus at 2.0 A Resolution.” 

Biochemistry (1998) 37, pgs 15981 – 15989. 

Protein images were created in PyMOL with PDB code 3THI