Why structural moiety prevents Dopamine from crossing the blood brain barrier?

Is it only lipid or non-lipid could go thru the Blood brain barrier? and which one is the one with the charge, lipid or non-lipid??
Answer:
The ability or inability of any chemical to cross the blood-brain barrier is dependent on its chemical structure. The drug levodopa is used as a prodrug to increase dopamine levels for the treatment of Parkinson's disease, since it is able to cross the blood-brain barrier whereas dopamine itself cannot.Levodopa, an amino acid that is a precursor of several neurotransmitters, enters and leaves the brain by means of the carrier for L-phenylalanine. Once in the endothelium, however, L-DOPA may be converted into dopamine and DOPAC in successive steps by the enzymes L-aromatic amino acid decarboxylase and MAO. Although dopamine can leave the brain by means of its own carrier, neither dopamine nor DOPAC can cross the antiluminal membrane into the brain. Hence the enzymatic conversion can serve as a means of controlling how much L-DOPA reaches the brain. However, conversion to dopamine also occurs in the peripheral tissues, causing adverse effects and decreasing the available dopamine to the CNS, so it is standard practice to co-administer a peripheral DOPA decarboxylase inhibitor 鈥?carbidopa or benserazide 鈥?and often a catechol-O-methyl transferase (COMT) inhibitor.I hope this helps.Rick the Pharmacist

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It's a little more complicated than that. The blood brain barrier severely restricts what it allows in. It is true that hydrophobic compounds are allowed in easier than hydrophilic compounds. But also compounds over 500 daltons are restricted. While there is also active transport of other compounds such as sugar and some amino acids. Non-lipid compounds tend to be charged. And dopamine cannot cross the BBB, but L-dopa can.

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http://pharmrev.aspetjournals.org/cgi/co...There are detailed answers in the link above:Generally, small, nonionic, lipid-soluble molecules penetrate easily across the BBB whereas larger, water-soluble, and/or ionic molecules will less likely exhibit passive diffusional processes (Spector, 1977, 1990).

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