Leuco-Phenolphthalein
Definition
The reduced, colourless form of phenolphthalein used in the Kastle-Meyer reagent. It is oxidised back to the pink quinone form in the presence of active oxygen released by haemoglobin's peroxidase-like activity.
- Form
- Reduced, colourless phenolphthalein
- Used in
- Kastle-Meyer presumptive blood test
- Colour on reaction
- Pink (quinone form)
- Trigger
- Oxidation by haemoglobin peroxidase-like activity
Common questions
Why does the leuco form need to be oxidised to give a result?+
The colourless reduced form only becomes visible when it loses electrons and converts to the pink quinone structure, and haemoglobin's peroxidase-like activity supplies the oxidation step in the presence of hydrogen peroxide, which is why the test is timed against a colour change rather than read as a static colour.
Does a positive Kastle-Meyer result confirm human blood?+
No, the peroxidase-like reaction is presumptive only because other biological oxidants such as plant peroxidases and some metal ions can also oxidise leuco-phenolphthalein, so a positive result requires confirmatory testing before it is treated as blood, let alone human blood.
Related terms
- Confirmatory Test
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- False Positive
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- Haem Group
- The iron-porphyrin prosthetic group at the centre of haemoglobin. The iron atom cycles between oxidation states during the peroxidase-like reaction, and it...
- Peroxidase-Like Activity
- The catalytic behaviour of haemoglobin (and other haem-containing proteins) that mimics the enzyme peroxidase: splitting hydrogen peroxide into water and oxygen. The...
- Presumptive Test
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