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

下个针对多巴胺的治疗药物是COMT抑制剂；COMT，即儿茶酚胺-O-甲基转移酶，是可以降解多巴胺和左旋多巴的酶。

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

首先，在酪氨酶（TH）的帮助下，酪氨也转为左旋多巴被称为左旋多巴。

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；卡比多巴是能通过血脑屏障的多巴脱羧酶抑制剂。

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.

多巴胺本身能通过血脑屏障，但它的前体左旋多巴可以，旦进入脑中，左旋多巴就会被多巴脱羧酶转换为多巴胺，尤其是在残存的黑质纹状体神元内。

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，比如多巴胺类似物左旋多巴，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。

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起用于治疗，只是为了防止左旋多巴在中枢神系统外被分解，从而使更多左旋多巴进入脑中。

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酶，可以在左旋多巴穿过血脑屏障前将其转换为多巴胺；然后，通过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。