Effects of the electrode potential on a direct electron transfer (DET)-type bioelectrocatalysis of fructose dehydrogenase (FDH) at Au electrodes were investigated. Adsorbed FDH showed the highest DET activity at an adsorption potential (E-ad) around the point of zero charge (E-pzc). Since FDH stock solution contains 2-mercaptoethanol (ME) for stabilization, ME is partially bound to the Au electrode. However, the DET activity drastically decreased at E-ad >> E-pzc. Au oxide layer is formed at the positive potentials to hinder the interfacial electron transfer. In contrast, only slight decrease in the DET activity was observed at sufficiently negative Ead (<< E-pzc), where ME is reductively desorbed from the Au electrode, but co-exists in the solution. In contrast, when FDH and ME were adsorbed on Au electrodes at an open circuit potential and the FDH- and ME-adsorbed Au electrode was held at such a negative hold potential (Eho) in the buffer without ME, the DET activity drastically decreased. An addition of ME in the test solution prevented the decrease in the DET activity at the negative E-ho. These results indicate that ME close to adsorbed FDH plays a significant role in the stabilization of FDH adsorbed on Au electrodes. (C) 2015 Elsevier Ltd. All rights reserved.