This is exactly why levodopa is administered with carbidopa, a dopa decarboxylase inhibitor that isn't able to cross the blood-brain barrier.

这也就是左旋多巴要和卡比多巴共用的原因；卡比多巴是一种不能通过血脑屏障的多巴脱羧抑制剂。

Alright the next dopamine-directed class of therapies are inhibitors of COMT, catecholamine-O-methyltransferase, which is an enzyme that degrades dopamine and levodopa.

下一个针对多巴的治疗药物是COMT抑制剂；COMT，即儿-O-甲基转移，是一种可以降解多巴和左旋多巴的。

Once inside the brain, Levodopa is efficiently converted to dopamine thus supplementing depleted dopamine levels in the midbrain.

一旦进入大脑，左旋多巴就有效地转为多巴，从而补充体内的多巴中脑的多巴水平。

First, with the help of enzyme tyrosine hydroxylase (TH), tyrosine gets converted to L-dopa, also known as levodopa.

首先，在酪氨酸（TH）的帮助下，酪氨酸也转为左旋多巴被称为左旋多巴。

Dopamine happens to be one of those molecules that cannot freely pass through this barrier, however, Levodopa can, with a little bit of help.

多巴正好是那些不能自由通过这个屏障的分子之一，不过，左旋多巴可以，只要一点点帮助。

Motor symptoms respond to drugs used to treat Parkinson's disease, like levodopa, a dopamine analogue, while cognitive symptoms may improve with drugs for Alzheimer's disease, like donepezil, which increases acetylcholine availability.

针对运动障碍，可使用与帕金森病类似的药物，比如多巴类似物左旋多巴，而使用类似阿尔茨海默病的药物，可以改善认知症状，比如多奈哌齐，它能增加乙酰胆碱的供给。

Now, when it comes to side effects, Levodopa in combination with Carbidopa may cause nausea, loss of appetite, hypotension, mental disturbances, and discoloration of urine, sweat or saliva.

现在，说到副作用，左旋多巴联合卡比多巴可能引起恶心，食欲不振，低血压，精神失常，尿、汗或唾液变色。

COMT inhibitors like entacapone and tolcapone are only given with levodopa to prevent the enzyme from breaking it down outside the central nervous system, thus allowing more of it to enter the brain.

COMT抑制剂，例如恩他卡朋和托卡朋与左旋多巴一起用于治疗，只是为了防止左旋多巴在中枢神经系统外被分解，从而使更多左旋多巴进入脑中。

Another agent that is used in combination with Levodopa and Carbidopa is Entacapone, which inhibits peripheral COMT and thus just like Carbidopa, it prolongs the time that Levodopa is available to the brain.

另一种与左旋多巴和卡比多巴联合使用的药剂是恩他卡彭，它能抑制外周血细胞凋亡，因此就像卡比多巴一样，它延长了左旋多巴对大脑有效。

Peripheral dopa decarboxylase also exists, which can metabolize levodopa into dopamine before it gets through the blood brain barrier, and—via additional enzymes— metabolize it into other catecholamines like epinephrine, which can cause unwanted side effects like arrhythmias.

体内还有外周多巴脱羧，可以在左旋多巴穿过血脑屏障前将其转换为多巴；然后，通过其他将其转换为其他儿，如肾上腺素，它可以导致不良的副作用，如心律失常。

Dopamine itself can't cross the blood-brain barrier, but its precursor levodopa can, and once in the brain, levodopa is converted to dopamine by dopa decarboxylase, most importantly within the remaining nigrostriatal neurons.

多巴本身不能通过血脑屏障，但它的前体左旋多巴可以，一旦进入脑中，左旋多巴就被多巴脱羧转换为多巴，尤其是在残存的黑质纹状体神经元内。