Synthesis of serine-AMC-carbamate: a fluorogenic tryptophanase substrate

A new fluorogenic substrate for the pyridoxal 5'-phosphate-dependent enzyme tryptophanase is described. L-Serine, which is linked to 7-amino-4-methylcoumarin through an O-carbamoyl tether, serves as a substrate for the enzyme. The released moiety, 7-amino-4-methylcoumarin (AMC), can be detected by either absorbance (355 nm) or fluorescence (excitation 365 nm/emission 440 nm). Kinetic constants were measured using each of these techniques: Km = 85 +/- 20 microM, Vmax = 2.9 +/- 0.4 mumol/min/mg (fluorescence) and Km = 129 +/- 21 microM, Vmax = 3.1 +/- 0.3 mumol/min/mg (absorbance). The Vmax for serine-AMC-carbamate is approximately 1.9 times faster than that of the natural substrate, tryptophan. Using fluorescence detection, solutions containing 10(-3) units of activity could be routinely assayed.