Drug Elimination IV

Reduction reactions may occur at
oxygen or nitrogen atoms. Keto-oxygens
are converted into a hydroxyl
group, as in the reduction of the prodrugs
cortisone and prednisone to the
active glucocorticoids cortisol and prednisolone,
respectively. N-reductions occur
in azo- or nitro-compounds (e.g., nitrazepam).
Nitro groups can be reduced
to amine groups via nitroso and hydroxylamino
intermediates. Likewise, dehalogenation
is a reductive process involving
a carbon atom (e.g., halothane).
Methylations are catalyzed by a
family of relatively specific methyltransferases
involving the transfer of
methyl groups to hydroxyl groups (Omethylation
as in norepinephrine [noradrenaline])
or to amino groups (Nmethylation
of norepinephrine, histamine,
or serotonin).
In thio compounds, desulfuration
results from substitution of sulfur by
oxygen (e.g., parathion). This example
again illustrates that biotransformation
is not always to be equated with bioinactivation.
Thus, paraoxon (E600)
formed in the organism from parathion
(E605) is the actual active agent .