Similarity of kinetics of three types of myeloperoxidase from human leukocytes and four types from HL-60 cells.

Abstract K m values for H 2 O 2 and V max values for three types of myeloperoxidase (MPO) from human leukocytes (MPO-I, -II, and -III) and four types from human myeloid leukemia HL-60 cells (MPO-I A , -I B , -II, and -III) were determined. K m values of human leukocyte MPOs decreased with increasing pH from 4.4 to 6.2 and increased with increasing NaCl concentration from 0.025 to 0.14 m . There was no significant difference among K m values of leukocyte MPO-I, -II, and -III. NaBr also showed a tendency similar to that of NaCl with regard to the effects of pH and halide concentration on K m values. However, K m values in the presence of NaBr were lower than those in the presence of NaCl. Effects of pH and NaCl concentration on V max values of MPO-I, -II, and -III were also examined. V max values of MPO-I, -II, and -III were higher at pH 4.9 and 5.4 and increased with increasing NaCl concentration. In addition, no difference was observed between K m values of leukocyte and those of HL-60 cells. MPO-I B , the half-molecular-weight enzyme of HL-60 cells, also had the same K m values as the others. Furthermore, inhibition of the activities of seven MPOs of leukocytes and HL-60 cells by H 2 O 2 was similarly observed at concentrations above 1 m m at pH 5.4 but not at pH 4.4. These results indicate that there is no difference in the affinity to H 2 O 2 among leukocyte MPO-I, -II, and -III and HL-60 cell MPO-I A , -I B , -II, and -III.