Ron,
No I’m not surprised that homocysteine is associated with several neurological disorders. In fact it suggests that a common factor is behind all this. Such a factor could be the widening of the BBB. But homocysteine is correlated with more than just that. This gives rise to the idea that the course is some mechanism in homocysteine-metabolism.
Next heavy metals play the same role. They are associated with the BBB and with neurological disorders. You may conclude that damage of the BBB is the common factor, but that does not convince me yet. Both heavy metal intoxication and elevated homocysteine levels are associated with a large number of diseases that are not neurological.
What I’m looking for is fundamental process, a chemical reason that explains why the BBB is insulted by homocysteine or heavy metals or pesticides. There is a process that is involved in all this and it is called methylation. This is the attachment or substitution of a methyl group And the big difference between levodopa and carbidopa in this respect is not the -NH-NH2 group that levodopa hasn't, but the methylgroup that levodopa hasn’t.

Joop