A novel procedure for the measurement of D-isocitrate is described tha
t utilizes the fluorescence quenching of thionine by the reduced form
of the cofactor NADPH, which results from the isocitrate dehydrogenase
-catalyzed conversion of D-isocitrate to 2-oxoglutarate. The procedure
is termed substrate induced-quenching (SIQ) assay and results in a fl
uorescence-quenching constant (K-SIQ) of 4443 M-1 obtained from data c
ollected over the range of 0-500 mu M isocitrate.