Why can L-dopa cross the blood brain barrier, but dopamine can't? - USMLE Forums
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Old 06-16-2011
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Question Why can L-dopa cross the blood brain barrier, but dopamine can't?

Above, we have L-dopa, while below we have dopamine. My understanding is that the more polar and the greater the size of a molecule, the less likely it will be able to cross the blood brain barrier (BBB). However, it appears to me that L-dopa (with its additional carboxylic acid group) would be even more polar than dopamine. Why then, does L-dopa cross the BBB while dopamine doesn't?

Thanks as always,
Tig
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Old 06-16-2011
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This is just my speculation but I believe you have to look at the overall molecule for polarity

For Dopamine, you have a NH2 group on one side that is electropositive while you have two -OH groups on the other end which are electronegative. Thus the overall polarity is pointing toward the OH groups.

In Levadopa, there are two Oxygen's on each side so they essentially cancel each other out leaving the overall polarity less than Dopamine.

I was a chemistry major in college but that was a long time ago so I may be wrong. That's how I would think about it though
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Old 06-30-2011
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I'm not planning to sit the USMLE but stumbled on your question studying for my own med exams then fortuitously stumbled on what I think is the answer:

http://www.ncbi.nlm.nih.gov/pubmed/11011012

I hope this helps!
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The above post was thanked by:
Ardavan (09-14-2013)



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