Abstract
Using an enzyme preparation from rabbit kidney cortex, the amino acids 3,4‐dihydroxyphenylalanine and 5‐hydroxytryptophan were found to inhibit each other's enzymatic decarboxylation competitively. Competitive inhibitions of these reactions by m‐tyrosine, o‐tyrosine, and caffeic acid were also observed. After purification of an enzyme preparation on a column of DEAE cellulose a peak appeared which contained most of the dihydroxyphenylalanine decarboxylase activity present in the extract. In this peak also nearly all the 5‐hydroxytryptophan decarboxylase activity was found. The ratio of the two activities was practically the same throughout the peak. The data were in agreement with the assumption that dihydroxyphenylalanine and 5‐hydroxytryptophan were decarboxylated by one enzyme.

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