Figure 4. Effect of substrates on kinetic stability of recombinant carnitine palmitoyltransferase (CPT) II enzymes.

A and B show the heat inactivation of His₆-N-hCPT2 and His₆-N-hCPT2/S113L, respectively, at different temperatures supported by palmitoyl-CoA. C and D show the heat inactivation of His₆-N-hCPT2 and His₆-N-hCPT2/S113L, respectively, at different temperatures suppressed by l-carnitine. Black squares, blue circles, and red triangles represent values at 30°C, 40°C, and 45°C, respectively. Open and closed symbols show the results of experiments performed in the absence or in the presence of palmitoyl-CoA or l-carnitine, respectively. E and F present the stabilization of His₆-N-hCPT2 and His₆-N-hCPT2/S113L, respectively, against heat inactivation by preincubation with acyl-l-carnitines at 45°C. Black open squares, blue open circles, red open triangles, gray open inverted triangles, black closed diamonds, black closed squares, blue closed circles, red closed triangles, and gray closed inverted triangles represent acetyl-l-carnitine (C2), isobutyryl-l-carnitine (C4), valeryl-l-carnitine (C5), hexanoyl-l-carnitine (C6), octanoyl-l-carnitine (C8), decanoyl-l-carnitine (C10), lauroyl-l-carnitine (C12), myristoyl-l-carnitine (C14), and palmitoyl-l-carnitine (C16), respectively. Data are presented as time-dependent changes of natural-log–transformed relative activities. The experiments were performed as explained in the Methods section. S113L = the variant S113L; WT = wild-type.