Metabolism
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An adverse response to antimalarial medication -
Samuel
W is an African-American recruit to the army. He was given the antimalarial
drug primaquine, and suffered a delayed reaction with kidney pain, dark urine,
and low red blood cell counts that led to anaemia and weakness. Centrifugation
of a blood sample showed a low haematocrit, and the plasma was red coloured.
Similar acute haemolytic attacks have been observed, predominantly in men of Afro-Caribbean origin, in response to primaquine and a variety of other drugs, including dapsone, the antipyretic acetylphenylhydrazine, the antibacterial bactrim/septrim, sulphonamides and sulphones, whose only common feature is that they all undergo cyclic non-enzymic reactions in the presence of oxygen to produce hydrogen peroxide and a variety of oxygen radicals that can cause oxidative damage to membrane lipids, leading to haemolysis. Moderately severe infection can also precipitate a haemolytic crisis in susceptible people.
Why do you think that infection can lead to a haemolytic crisis in susceptible people?
Part of the response to infection is activation of macrophages to produce a mixture of oxygen and other radicals that are cytotoxic to engulphed micro-organisms. This leads to an increase in whole body radical burden, and increased oxidative damage to cell membranes.
One way of screening for sensitivity to primaquine is based on the observation that the glutathione concentration of erythrocytes from sensitive subjects is somewhat lower than that of control subjects, and falls considerably on incubation with acetylphenylhydrazine.
Glutathione (GSH) is a tripeptide, gamma-glutamyl-cysteinyl-glycine, which readily undergoes oxidation to form a disulphide-linked hexapeptide, oxidised glutathione, generally abbreviated to GSSG
The table below shows the concentrations of GSH and GSSG in red cells from 10 control subjects, and Samuel W, before and after incubation with acetylphenylhydrazine.
The effect of incubation with 330 µmol /L acetylphenylhydrazine on erythrocyte glutathione.
controls |
Samuel W |
|||
GSH mmol /L |
GSSG µmol /L |
GSH mmol /L |
GSSG µmol /L |
|
| initial | 2.01 ± 0.29 |
4.2 ± 0.61 |
1.61 |
400 |
| + acetylphenylhydrazine | 1.82 ± 0.24 |
190 ± 28 |
0.28 |
1540 |
What conclusions can you draw from these results?
How much glutathione has been oxidised per mol of acetylphenylhydrazine added?
Samuel’s red blood cells contain significantly less GSH than normal, and a very high level of GSSG.
The results suggest that acetylphenylhydrazine causes the oxidation of glutathione to GSSG. This is not a simple stoichiometric oxidation of glutathione by acetylphenylhydrazine. In Samuel's erythrocytes, the ratio of glutathione oxidised : acetylphenylhydrazine present = 4.7. This suggests that it is likely that acetylphenylhydrazine undergoes a cyclic redox reaction that results in the production of hydrogen peroxide, which in turn oxidises glutathione.
In control subjects incubation with acetylphenylhydrazine leads to a modest decrease in GSH, and a small accumulation of GSSG; Samuel’s red cells show a very considerable depletion of GSH and a very large accumulation of GSSG on incubation with acetylphenylhydrazine.