Levodopa can cross the BBB and is less lipid soluble then dopamine, so dopamine should be able to cross the BBB more easily then l-dopa, but for some reason dopamine cant cross the BBB. Both dl-methamphetamine and levomethamphetamine can cross the blood brain barrier, but why not dl-dopamine?
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Both L-Dopa and Dopamine are polar molecules and thus hydrophilic. They are solvable in water but not in lipids, so they are not able to cross the Blood-brain barrier on their own.
The difference is that L-Dopa has a transport protein (SLC7A5) that allows it to pass through the Blood-brain barrier, whereas dopamine doesn't.
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$\begingroup$ Yeah, that's LAT1, but why doesn't DAT1 have any effect? $\endgroup$ Apr 25, 2017 at 10:18
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$\begingroup$ If you look at the distribution of LAT1 versus the distribution of DAT you can see that LAT1 is distributed in the blood-brain barrier, whereas DAT is not. $\endgroup$– maccoApr 25, 2017 at 10:42