Fig. 1: MetCa and MetCb together function as a Ni²⁺-dependent metformin hydrolase.

a Schematic representation of metformin hydrolysis and the putative genes involved. DMA, dimethylamine. b Conversion of metformin by cell extracts of E. coli expressing MetCaCb alone or together with MetAB in the presence of various transition metal ions. The cell extracts were incubated with 10 mM metformin and 0.2 mM of the indicated metal ions, and guanylurea production was measured after 12 h of incubation. c Metformin hydrolase assays with purified MetCa, MetCb and co-purified MetCaCb. The components of the reaction mixtures are indicated below each column. The arginase inhibitor 2(S)-amino-6-boronohexonic acid (ABH) and the nickel (Ni)-specific chelator dimethylglyoxime (DMG) were tested for their effects on the activity of MetCaCb. All assays in (b) and (c) were performed in three independent preparations. Data are presented as mean values ± SD. d ¹³C-NMR spectra of ¹³C¹⁵N-labeled metformin (10 mM) reaction mixtures with or without recombinant MetCaCb. e Growth phenotypes of strain NyZ550 and its gene-knockout variants on 5 mM glucose (GLU), metformin (MET), and dimethylamine (DMA). The respective gene complementation and their growth phenotypes are indicated within parentheses. MET and DMA served as the sole sources of carbon and nitrogen, while GLU was used as the sole carbon source with ammonium sulfate as nitrogen source. The “+” means that the value of OD_600nm was >0.1 at the stationary phase, which represents a more than 10-fold increase of the OD_600nm. The “−” means there was no evident increase of the OD_600nm observed at an initial inoculum of 0.01.