To test this hypothesis, we evaluated the enzymatic activity of wild-type HDC, the 35-kD mutant, and the combination of the two. A fluorescence-quenching–based approach confirmed that the mutant form lacked enzymatic function (Fig. 3C). We subsequently used liquid chromatography–mass spectrometry to determine the dose–response characteristics of histamine production when various ratios of wild-type and mutant HDC were combined,11 and we found evidence for a dominant negative effect of the mutant protein (Fig. 3E and 3F).
