Indicator assay for amino acid decarboxylases.

Indicator assays have been devised for glutamate decarboxylase and arginine decarboxylase as models for a general procedure for amino acid decarboxylases. Since the decarboxylation results in the absorption of a proton in the pH range of maximum enzymatic activity, the change in absorbance of an acid-base indicator can be used to follow the progress of the reaction. For glutamate decarboxylase, the substrate was used as the buffer, and 1-1'-diethyl-2-2'-cyanine iodide was used as the indicator. For arginine decarboxylase, acetate was used as the buffer, and bromcresol green was used as the indicator. The change in absorption with extent of reaction is linear if the indicator pK is equal to the buffer pK.