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经元内。

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，它可以导致不良的副作用，如心律失常。