Some thirty years ago, as 2nd year medical students, a couple of us were sat outside our ramshackle hostel in the gathering dusk. Biochemistry exams were in the offing and we were quizzing each other just to make sure that we were all keeping up. Somebody piped up about the importance of not eating raw eggs. The prevailing dogma, which we all agreed upon then, was that biotin in the egg yolk binds to an egg protein called avidin. Cooking the egg broke the bond, which made biotin available to the body, where it subserved carboxylation reactions as a co-enzyme to acetyl co-A carboxylase and pyruvate carboxylase.
As it happens, we were wrong. I succumbed to atavism and purchased the latest edition of Harper's Biochemistry a few years ago. Biotin deficiency is virtually unknown in medicine. You would have to be a habitual eater of raw egg whites, eschewing the yolk, to get even close to being Biotin deficient.
However, elsewhere in the world, some other people took note of the affinity between biotin and avidin and thought of an application that would change immuno-assays for generations to come. It thus came about that biotin is used as a tag. Biotin has a valeric acid side chain that can be modified to make it reactive and conjugate it to almost any protein or nucleic acid. By then adding avidin or an avidin like bacterial molecule called Streptavidin, produced by Streptomyces avidinii to the mixture, you can home in to the protein that has been bound to biotin. Of course, you need to find where the avidin goes. So you label the avidin (or streptavidin) with a "dye" (or a fluorophore, to be more technical), or an enzyme, which will change colour once its substrate is added. And presto, by taking advantage of one of the strongest known non-covalent affinities in nature, every biotin molecule, attached to the protein that you seek to find, will be picked out like a guided missile by the avidin.
The biotin-streptavidin immunossay has become the standard in most labs for procedures such as ELISA, immunohistochemistry etc.
You can do the same thing to find RNA or DNA, known as Northern blotting or Southern blotting. Here, avidin is tagged to a nucleic acid probe that hybridises with the DNA or RNA bound by biotin. However, these blotting procedures have largely been supplanted by PCR.
But that's not the end of the story. Over the recent years, biotin has become quite a popular supplement for people with mitochondrial disorders and in general as a "pick up" as a component of multivitamins. It definitively found its way into mainstream medicine when trials showed that mega-doses of biotin had salutary effects on the functioning of subjects with multiple sclerosis. It is now increasingly prescribed for MS.
Couple of years ago, reports started filtering in of bizarre lab results, particularly TFTs, but also other endocrine tests such as testosterone, FSH and LH and vitamin B12 among others, in subjects who were on biotin supplements. Remarkably the free T3 and T4 would be very high, the TSH suppressed, and levels of TRAb would be high- an exact biochemical mimic of Grave's disease....except that these subjects were completely euthyroid.
It is now appreciated that pharmacologic doses of biotin interferes with the biotin-streptavidin assay. In competitive immunoassays, the substances being measured such as free T3, or free T4, will be falsely high, whereas substances measured in "sandwich immunoassays", such as TSH, PTH and PSA, will be very low.
There have been a number of instances where subjects on biotin supplements have been erroneously treated for Graves Disease, based on such lab results. There are two excellent letters in the latest edition of New England Journal of Medicine that expand on these. They are free to access. One can avoid such misrepresentation of lab results by stopping the biotin supplements 72 hours before running TFTs or other blood tests.
I shared this as it took me back three decades to that long forgotten, dilapidated hostel where we grew up as wide-eyed medical students, completely unaware of how things were about to explode from bench to bedside.
References:
1.Kummer S, Hermsen D, Distelmaier F. Biotin treatment mimicking Graves’ disease. N Engl J Med 2016;375:704-706
2. Trambas CM, Sikaris KA, Lu ZX. A caution regarding high-dose biotin therapy: misdiagnosis of hyperthyroidism in euthyroid patients. Med J Aust 2016;205:192-192
No comments:
Post a Comment