If the enzyme is sufficiently represented in the cerebro-spinal fluid (CSF), then all you need to do is take a sample of that. Lumbar punctures/spinal taps aren't without risk, but they are considered safe [0]. If the enzyme is not well represented in the CSF, then you'd likely need a tissue sample. You may get lucky and find that the enzyme is represented in the meninges (the membranes that separate your brain from your skull) and take a sample of that [1]. This would not be easy and would have more complications than a spinal tap, of course. If you still don't have good representation of the enzyme, then what's left is an actual brain tissue sample. Likely, you'd take something from the parietal lobe, to minimize cognitive losses and standard of living issues.
Honestly, all these forms of sample recovery are absurdly unethical and would never pass an IRB. Since the enzyme is only being used to 'enhance' people, it's very unlikely you'd ever be allowed to get samples like these. Urine, sure. Saliva, sure. Blood, unlikely. CSF, never. The core issue is that since no harm is being done via these enzymes (supposedly) you can't introduce further harm that these samples require to obtain them. THe ethical issues are pretty black-and-white.
If the enzyme is sufficiently represented in the cerebro-spinal fluid (CSF), then all you need to do is take a sample of that. Lumbar punctures/spinal taps aren't without risk, but they are considered safe [0]. If the enzyme is not well represented in the CSF, then you'd likely need a tissue sample. You may get lucky and find that the enzyme is represented in the meninges (the membranes that separate your brain from your skull) and take a sample of that [1]. This would not be easy and would have more complications than a spinal tap, of course. If you still don't have good representation of the enzyme, then what's left is an actual brain tissue sample. Likely, you'd take something from the parietal lobe, to minimize cognitive losses and standard of living issues.
Honestly, all these forms of sample recovery are absurdly unethical and would never pass an IRB. Since the enzyme is only being used to 'enhance' people, it's very unlikely you'd ever be allowed to get samples like these. Urine, sure. Saliva, sure. Blood, unlikely. CSF, never. The core issue is that since no harm is being done via these enzymes (supposedly) you can't introduce further harm that these samples require to obtain them. THe ethical issues are pretty black-and-white.
[0] https://www.mayoclinic.org/tests-procedures/lumbar-puncture/...
[1] https://en.wikipedia.org/wiki/Meninges