3. Physiology
        3.6. Haematology
            3.6.1. Blood cells
                3.6.1.3. Red blood cells
                    3.6.1.3.1. Haemoglobin
3.6.1.3.1.1. Methaemoglobin

Methaemoglobin (MetHb)

[Ref: JN5:p272-273; WG21:p536-539; KB:p211]

In MetHb,

Iron is ferric (Fe3+) instead of ferrous (Fe2+)

Characteristics

Formation

Reduction

Iron is reduced back to ferrous
--> MetHb converted back to Hb

By 4 systems:

1. NADH-methaemoglobin reductase system

2. Ascorbic acid

May slowly reduce the iron by direct chemical effect

Accounts for 16% of MetHb reduction

3. Glutathione-based reductive enzymes

Small amount of MetHb reductase activity

4. NADPH-dehydrogenase enzyme in RBC

 

NB:

[KB2:p137]

In presence of methylene blue, activity of NADPH-MetHb reductase (?? same as NADPH-dehydrogenase) increases
--> NADPH-MetHb reductase reduces methylene blue
--> Reduced form of methylene blue reduces MetHb

 

 



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