Now for the complications. (Aren't there always complications
in life?) The final reaction to the neurotransmitter in both the
case of dopamine and serotonin, is decarboxylation, and the same
enzyme (the aromatic L-amino acid decarboxylase) is involved in
both conversions. This decarboxylase enzyme is present in the
liver, and it acts in the case of L-DOPA to convert the compound
to dopamine before it can make it into the brain (and if this
happens, the L-DOPA is wasted). The decarboxylase enzyme uses B6
as a cofactor for this reaction, and for this reason a
Parkinson's disease patient taking L-DOPA cannot take more than
the RDA of B6, because doing so would act to neutralize
oral L-DOPA too quickly. These days, almost all Parkinson's
patients on L-DOPA take the drug in a combination with an
artificial decarboxylase inhibitor, called Carbidopa (the
combination is called Sinemet). But even with Carbidopa,
Parkinson's patients are advised not to exceed a daily dose of B6
of 25 mg, since more will overwhelm the Carbidopa effect, and
cause pharmacologic L-DOPA to be destroyed in the liver before it
can get into the brain.