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G6PD

Glucose-6-phosphate dehydrogenase (G6PD or G6PDH) is an enzyme which reduces nicotinamide adenine dinucleotide phosphate (NADP) to NADPH while oxidizing glucose-6-phosphate.  People who have a genetic deficiency of G6PD, also known as favism, are predisposed to an allergic-type non-immune haemolytic anaemia (spontaneous destruction of red blood cells).
G6PD has many genetic polymorphisms. Many variants of G6PD, mostly produced from missense mutations, have been described with wide ranging levels of enzyme activity and clinical symptoms. 

Two variants (G6PD A− and G6PD Mediterranean) are the most common in humans. G6PD A− has an incidence of 10% of American blacks, while G6PD Mediterranean is prevalent in the Middle East. The known distribution of the disease is largely limited to people of Mediterranean origins (Spaniards, Italians, Greeks, Armenians, and Jews). These variants are believed to stem from a protective effect against Plasmodium falciparum and Plasmodium vivax malaria.

All mutations that cause G6PD deficiency are found on the long arm of the X chromosome, on band Xq28. Favism is therefore more common in males.

Glucose-6-phosphate dehydrogenase deficiency can cause acute haemolytic anaemia in the presence of simple infection, eating broad beans (fava beans) due to alkaloids in the beans, or reaction with certain medicines such as antibiotics, antipyretics, and antimalarials.

People with reactions to broad beans or medicines such as aspirin, certain antimalarials and antibiotics may have a form of G6PD deficiency. High dose oral or intravenous vitamin C or vitamin K3 may also cause haemolytic anaemia in people with favism. It is generally considered safe to take up to 6 grams of vitamin C without side-effects, but higher doses should be used with caution in case of favism. G6PD levels can be tested in the blood, and the G6PD levels of people with favism is generally low. 

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"Broad-beans-after-cooking". Licensed under Public domain via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Broad-beans-after-cooking.jpg#mediaviewer/File:Broad-beans-after-cooking.jpg