carbidopa confusion

[iki4life]

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2 questions-

1. cofactor for dopa--->dopamine is b6 or bh4?….FA says b6, UW says bh4.
2. why is carbidopa given in parkinsons when it blocks dopa decarboxylase, the enzyme that makes dopamine..and parkinsons is a disease of low dopamine anyway? i understand carbidopa lets levodopa bypass the effects of b6 in parkinsons but it also inhibits dopamine synthesis, no? is carbidopa only given to enhance levodopa's effect of letting dopamine cross the BBB even though carbidopa itself will decrease dopamine synthesis?

i hope this question makes sense, its a bit confusing.

thanks

 

[Mr. Mojo Risin]

Levodopa can cross the BBB, but dopamine cannot. You want dopamine in the brain, so you give carbidopa (a peripheral DDC inhibitor) with levodopa to prevent the peripheral conversion (via DDC) of levodopa to dopamine before levodopa has the chance to reach the BBB. Carbidopa only affects peripheral DDC because, like dopamine, it cannot cross the BBB. Thus, carbidopa does not inhibit central dopamine synthesis.

As for biosynthesis/According to FA, phenylalanine to tyrosine requires BH4, tyrosine to dopa requires BH4, dopa to dopamine requires B6.

 

[iki4life]

Levodopa can cross the BBB, but dopamine cannot. You want dopamine in the brain, so you give carbidopa (a peripheral DDC inhibitor) with levodopa to prevent the peripheral conversion (via DDC) of levodopa to dopamine before levodopa has the chance to reach the BBB. Carbidopa only affects peripheral DDC because, like dopamine, it cannot cross the BBB. Thus, carbidopa does not inhibit central dopamine synthesis.

As for biosynthesis/According to FA, phenylalanine to tyrosine requires BH4, tyrosine to dopa requires BH4, dopa to dopamine requires B6.

thank you…helps a lot!