The interaction of a-ketoglutarate dehydrogenase complex (α-KGDC) with a hydrophobic fluorescent probe [1,1`-bi(4-aniline)naphthalene-5,5`-disulfonic acid] (bis-ANS) was studied. The purified α-KGDC was potently inhibited by bis-ANS with an apparent half maximal inhibitory concentration (IC_(50)) of 9.8 μM at pH 8.0. The catalytic activities of both the E1o and E2o subunits were predominantly inhibited while that of the E3 component was hardly affected. The binding of bis-ANS to the enzyme caused a marked enhancement and blue shift from 523 ㎚ to 482 ㎚ in the fluorescence emission spectrum. The dissociation constant (Kd) and the number of binding sites (n) were calculated to be 0.87 mM and 158, respectively. Allosteric regulators such as purine nucleotides and divalent cations further increased the fluorescence intensity of the bis-ANS-α-KGDC binary complex. These data suggest that the binding of these allosteric regulators to α-KGDC may cause the conformational changes in the enzyme and that bis-ANS could be used as a valuable probe to study the interaction of the multi-enzyme complex and its allosteric regulators.