Friday, October 5, 2012


What is Methemoglobin?

Methemoglobin is a form of hemoglobin in which the iron in the heme group is in the Fe3+ (ferric) state.
Normally, it is in the Fe2+ (ferrous) state. So it is an oxidized form of Hb (hemoglobin).
What is wrong with that?
Methemoglobin cannot bind to oxygen :(

The NADH-dependent enzyme methemoglobin reductase (diaphorase I) converts methemoglobin back to hemoglobin (yaay!) Due to this enzyme, only one to two percent of your hemoglobin is methemoglobin.

Deficiency of this enzyme causes congenital methemoglobinemia.

Cool fact: It is bluish chocolate-brown in color :D
In fact, this is used to detect the concentration of hemoglobin by colorimetery.

Drabkin’s Solution reacts with all forms of hemoglobin (except sulfhemoglobin)
So you basically oxidize hemoglobin and its derivatives to methemoglobin in the presence of alkaline potassium ferricyanide. Methemoglobin reacts with potassium cyanide to form cyanmethemoglobin, which has maximum absorption at 540 nm. The color intensity measured at 540 nm is proportional to the total hemoglobin concentration :)

Also, the amount of cyanide present in the reagent is appreciably less than the lethal dose (So you can never commit suicide by ingestion of the reagent. It was asked in my viva! :P )

Certain drugs can also cause methohemoglobinemia like benzocaine, chloroquine, dapsone, nitrates, nitrites, nitroglycerin, nitroprusside, primaquine, quinones and sulfonamides :O

Methemoglobin comes to the rescue in cyanide poisoning! ^_^
Cyanide ion halts cellular respiration by inhibiting an enzyme in mitochondria called cytochrome c oxidase. Methemoglobin has high affinity for cyanide radical and forms cyanomethemoglobin.

But how will we form methoemoglobin in the first place? @_@
If you're thinking one of the drugs mentioned above, you're on the right track. We use sodium nitrite (A weak vasodilator compared to the other nitrites). Also, cyanomethemoglobin may dissociate to release cyanide again. That's why, we also administer sodium thiosulfate which forms sodium thiocyanate which is poorly dissociable and excreted in urine.

Methemoglobin reminds me of how they say, "A stopped clock is right twice a day" (:

Methylene blue can also be used in methemoglobinemia.

Another cool fact: Ingestion of Paracetamol causes methemoglobinemia in cats :O

That's all ^_^


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