The metabolites of steroid hormones catalyzed by cytochrome P450s (CYPs) play a significant role in carcinogenicity. Herein we report a facile electrochemical liver microsomal bioreactor based on the polydopamine encapsulated gold-graphene nanocomposite (PDA/Au@RGO) for the fast steroid hormone metabolism assays. In this work, human liver microsomes (HLMs) were directly attached on PDA/Au@RGO without any activation process. Besides, due to the high conductivity, large active areas and good biocompatibility of PDA/Au@RGO, the direct electron transfer of HLM with the rate constant (k(s)) of 8.9 s(-1) was achieved and the bioactivity of membrane bound proteins in HLM was largely maintained. Compared with recombinant enzymes, the immobilized HLM displayed obviously lower apparent Michaelis-Menten constant (K-m(app)) of 175 +/- 20 mu M. Through HPLC-MS and CYP-specific inhibition experiment, the electrode-driven catalytic behavior of HLM for the conversion of testosterone to 6 beta-OH-testosterone was confirmed. Moreover, the constructed HLM bioreactor was also successfully applied to the metabolism of other two steroid hormones (estrone and progesterone). This work has immense significance in the design of practically viable HLM bioreactors for the toxicity analysis of steroid hormones and stereoselective metabolite synthesis. (C) 2017 Elsevier Ltd. All rights reserved.