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

这也就是左旋要和卡共用的原因；卡是种能通血脑屏障的脱羧酶抑制剂。

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

下个针对胺的治疗药物是COMT抑制剂；COMT，即儿茶酚胺-O-甲基转移酶，是种可以降解胺和左旋的酶。

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

旦进入大脑，左旋就有效地转化为胺，从而补充体内的胺中脑的胺水平。

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

首先，在酪氨酸羟化酶（TH）的帮助下，酪氨酸也转化为左旋被称为左旋。

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

胺正好是那些能自由通这个屏障的分子之，，左旋可以，只要点点帮助。

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，如胺类似物左旋，而使用类似阿尔茨海默病的药物，可以改善认知症状，如奈哌齐，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，左旋联合卡可能引起恶心，食欲振，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起用于治疗，只是为了防止左旋在中枢神经系统外被分解，从而使更左旋进入脑中。

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.

另种与左旋和卡联合使用的药剂是恩他卡彭，它能抑制外周血细胞凋亡，因此就像卡样，它延长了左旋对大脑有效。

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.

体内还有外周脱羧酶，可以在左旋穿血脑屏障前将其转换为胺；然后，通其他酶将其转换为其他儿茶酚胺，如肾上腺素，它可以导致良的副作用，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.

胺本身能通血脑屏障，但它的前体左旋可以，旦进入脑中，左旋就被脱羧酶转换为胺，尤其是在残存的黑质纹状体神经元内。