Why structural moiety prevents Dopamine from crossing the blood brain screen?
Answer:
The ability or inability of any chemical to cross the blood-brain railing 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 competent to cross the blood-brain barrier whereas dopamine itself cannot.
Levodopa, an amino acerbic that is a precursor of several neurotransmitters, enter and leaves the brain by means of the mover for L-phenylalanine. Once in the endothelium, however, L-DOPA may be converted into dopamine and DOPAC contained by successive steps by the enzymes L-aromatic amino acid decarboxylase and MAO. Although dopamine can go the brain by means of its own delivery service, 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 reach the brain.
However, conversion to dopamine also occurs contained by the peripheral tissues, cause adverse effects and decreasing the available dopamine to the CNS, so it is standard practice to co-administer a peripheral DOPA decarboxylase inhibitor – carbidopa or benserazide – and repeatedly a catechol-O-methyl transferase (COMT) inhibitor.
I hope this helps.
Rick the Pharmacist
It's a touch more complicated than that. The blood brain barrier severely restricts what it allows surrounded by. It is true that hydrophobic compounds are allowed in easier than hydrophilic compounds. But also compounds over 500 daltons are restricted. While here 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.
http://pharmrev.aspetjournals.org/cgi/co...
There are detailed answers within the link above:
Generally, small, nonionic, lipid-soluble molecules permeate easily across the BBB whereas larger, water-soluble, and/or ionic molecules will smaller amount likely exhibit submissive diffusional processes (Spector, 1977, 1990).