Aromatic acid decarboxylase and choline acetylase activities in a single identified 5‐HT containing cell of the leech

The two largest cells (Retzius cells) in a typical ganglion of the leech nervous system have been shown previously to contain a high concentration of 5‐hydroxytryptamine (5‐HT). The present study demonstrates that the isolated Retzius cell somata contain approximately 40% of the total aromatic acid decarboxylase activity of a single ganglion. A single Retzius cell has approximately 70 times more enzyme activity than adjacent “control” cells. Since this enzyme activity subserves the synthesis of 5‐HT, a metabolic basis is provided for the cell‐specific accumulation of the amine in the Retzius cells. Further indication of the specificity of the above metabolic relationship was obtained from the study of the distribution of choline acetylase, the enzyme responsible for the synthesis of acetylcholine. The specific activity (enzyme activity per unit of protein) of the acetylase in the “control” cells is nearly 8 times greater than that in the Retzius cell.