Fig. 6

Effects of pH (a) and temperature (b) on VDH activity (●) and stability (○). The optimal pH was measured at 30 °C in buffer solutions of distinct pH under otherwise standard assay conditions. The buffer systems used were sodium acetate buffer (pH 4.0–5.5), sodium phosphate buffer (pH 6.0–7.5), Tris-HCl buffer (pH 8.0–9.0), and glycine-NaOH buffer (pH 9.5–11.0). The maximal activity, obtained at pH 9.5, was defined as 100%. The pH stability of VDH was determined by preincubating the enzyme in each buffer at 4 °C for 15 h and measuring the residual activity by using the standard assay, with the initial activity regarded as 100%. The optimal temperature was determined by measuring enzyme activity at various temperatures under otherwise standard assay conditions; the maximal activity, obtained at 45 °C, was defined as 100%. Lastly, thermal stability was determined by preincubating the enzyme at various temperatures for 10 min in sodium phosphate buffer (pH 7.0) and measuring the residual activity by using the standard assay method; the activity measured without preincubation was regarded as 100%. Data are shown as means ± standard deviation (error bars; n = 3)