NADH inhibits Sir2 NAD-dependent histone deacetylase activity. (A) Kinetic analysis of the Sir2 NAD-dependent histone deacetylase activities in the presence of 0 μM (filled squares), 50 μM (open squares); 100 μM (filled circles), or 250 μM (open circles) NADH. Here 150 ng of recombinant GST-tagged yeast Sir2 protein was assayed with various concentrations of NAD and NADH for 2.5 h at 30°C. Data are shown as a Lineweaver-Burk double reciprocal plot of 1/V (CPM/h) versus 1/[NAD] (μM). The results show the average of three independent experiments, each measured in duplicate. (B) Kinetic analysis of the human SIRT1 NAD-dependent histone deacetylase activities in the presence of 0 μM (filled squares), 50 μM (open squares), 150 μM (filled circles), or 300 μM (open circles) NADH. Experiments were carried out as in A, except that 50 ng of recombinant GST-tagged human SIRT1 protein (Takata and Ishikawa 2003) was used, and the reaction was carried out for 2 h at 37°C.