Abnormal neutrophil myeloperoxidase from a patient with chronic myelocytic leukemia

Neutrophil myeloperoxidase from a patient with chronic myelocytic leukemia was isolated under conditions designed to minimize proteolysis. Those methods yielded an α₂β₂ form of myeloperoxidase from normal individuals. Purified enzyme from the patient had electronic absorbances ($\text{A}{}_{430}\text{A}{}_{280}\text{= 0.85}$), enzymatic activity, and electrophoretic and Chromatographic behavior indistinguishable from that of normal myeloperoxidase. Edman degradation and physical studies after reduction and denaturation, however, showed that as compared to normal enzyme, one 55,000-dalton α subunit of the patient's myeloperoxidase was replaced by a 39,000-dalton peptide with a different amino-terminal sequence, a mixture of smaller peptides, and an heme derivative. Myeloperoxidase from the leukemic neutrophils appeared to have been partially degraded in vivo by lysosomal proteases.