5. The kinetic parameters

of α-IPMS-2CR and α-IPMS-14CR for both substrates are summarized in Table 1. The selleck kinase inhibitor apparent Km and Vmax of α-IPMS-2CR do not agree with those reported previously (Km and Vmax for α-ketoisovaleric acid was 24.6 μM and 0.8 U/mg, respectively; Km and Vmax for acetyl CoA were 243.5 μM and 2.07 U/mg, respectively) [4]. The reason for these discrepancies is unclear, but may be at least partially due to differences in enzyme preparation and storage conditions. In the previous report, the enzyme was maintained in an elution buffer containing 100–250 mM imidazol, while in this report, dialysis was performed to eliminate imidazol from the enzyme solutions and purified protein fractions obtained by gel filtration were used in the assays. Table 1 Kinetic parameters, Vmax and Km, of α-IPMS reacting to α-ketoisovaleric acid and acetyl click here CoA a α-IPMS α-Ketoisovaleric acid Acetyl CoA   Km (μM) Vmax (U/mg protein) R2 k cat b (s-1) k cat /Km (s-1 M-1) Km (μM) Vmax (U/mg protein) R2 k cat b (s-1) k cat /Km (s-1 M-1) α-IPMS-2CR 261 (S.E. = 14.7) 0.49 (S.E. = 0.01) 0.99 1.17 4480 568 (S.E. = 94.5) 0.93 (S.E. = 0.06) 0.99 2.22 3,900 α-IPMS-14CR 35 (S.E. = 5.4) 0.16 (S.E. = 0.01) 0.96 0.52 14,800 27 (S.E. = 6.9) 0.19 (S.E. = 0.01) 0.93 0.61 22,590 a At pH 8.5 and 37°C. The apparent Km and Vmax values were determined by varying the concentrations of one substrate at a fixed saturating concentration of the other substrate.

α-ketoisovaleric acid and acetyl CoA was fixed at 2 mM and 0.8 mM, respectively. Data are the LCZ696 manufacturer average of two assays. Prism software (version 3.08) was used for nonlinear regression, curve fit analysis to calculate Km and Vmax. b k cat = Vmax/[E] (μmol s-1mg-1)/(mol mg-1) Comparison of the apparent Km/Vmax of α-IPMS-2CR and α-IPMS-14CR, processed through similar conditions, shows that α-IPMS-2CR has a lower affinity for its substrates than α-IPMS-14CR (4-fold lower for α-ketoisovaleric

acid and 14-fold lower for acetyl CoA). The Vmax values for both substrates of α-IPMS-2CR were higher than those of α-IPMS-14CR, resulting in a higher k cat . α-IPMS-14CR has a higher catalytic efficiency, however, as k cat /Km ratios for α-ketoisovaleric acid and acetyl CoA were approximately 2 and 5 times higher, respectively, than those of α-IPMS-2CR. The l-leucine feedback inhibition of α-IPMS was investigated with the addition ASK1 of 0.1 to 10.0 mM l-leucine to the enzyme assay mixtures. The inhibition of α-IPMS-2CR was clearly detectable in the presence of 0.4 mM l-leucine, and the enzyme was inhibited by almost 50% with 0.8 mM l-leucine. l-leucine had no significant effect on α-IPMS-14CR activity under similar assay conditions (Figure 4).