The drug acts by inhibiting which of the following enzymes?

A 68-year-old man presents for a follow up visit in clinic.
He presented six months ago with short term memory loss, loss of verbal fluency and dysphasia. His wife then
reported that he has become ‘less tolerant’ and ‘overreacts’. Examination at that point was normal and he
scored 25/30 on the mini mental state examination.
A CT brain was essentially normal. Full blood count, biochemical tests, thyroid function, B12 and folate were all normal. He scored 19/30 on a repeat MMSE today. He is diagnosed as having probable Alzheimer’s disease and is commenced on donepezil.
The drug acts by inhibiting which of the following enzymes?

A Catechol-O-methyl transferase
B Cholinesterase Correct
C Dopamine decarboxylase
D Glutamic acid decarboxylase
E Monoamine oxidase

Donepezil is a cholinesterase inhibitor. It increases acetylcholine levels in the brain and has been shown to
slow progression of Alzheimer’s dementia.
Answer A is incorrect because catechol-O-methyl transferase is responsible for degradation of dopamine,
adrenaline and noradrenaline and catechol-O-methyl transferase inhibitors are used in the treatment of
Parkinson’s disease.
Answer C is incorrect because dopamine decarboxylase degrades dopamine and dopa decarboxylase
inhibitors are used in the treatment of Parkinson’s disease.
Answer D is incorrect because glutamic acid decarboxylase is responsible for converting glutamic acid into
gamma-aminobutyric acid which is an inhibitory neurotransmitter. Inhibition of this enzyme is likely to give rise
to uncontrollable movements such as chorea.
Answer E is incorrect because monoamine oxidase breaks down most cerebral neurotransmitters. Monoamine
oxidase inhibitors can be used in the treatment of depression.