Processing of a newly identified intermediate of human myeloperoxidase in isolated granules occurs at neutral pH.

Myeloperoxidase is a major component of specialized lysosomes known as azurophil granules in polymorphonuclear leukocytes or neutrophils. The processing of myeloperoxidase in human HL-60 promyelocytic leukemia cells was studied by pulse-labeling cells in culture with [35S]methionine followed by immunoprecipitation and identification of myeloperoxidase polypeptides from cell fractions after various chase intervals. These studies revealed the presence of a previously unidentified intermediate with Mr 74,000 which kinetically followed the appearance of a larger Mr 81,000 intermediate. Using an in vitro lysosomal preparation the newly identified Mr 74,000 intermediate was directly converted within protected granules to mature forms of myeloperoxidase (Mr 63,000 and 60,000). This conversion occurred optimally at pH 7.5 and was not inhibited by lysosomotropic agents (chloroquine, NH4Cl) or protonophores (monensin, carbonyl cyanide p-trifluoromethoxyphenylhydrazone). Furthermore, the uptake of radiolabeled amines indicated a neutral intragranular environment (pH 7.35-7.67) which remained unchanged in the presence and absence of 1 mM ATP or 2.5 microM carbonyl cyanide p-trifluoromethoxyphenylhydrazone. We conclude that, in contrast to other lysosomal pathways, the final proteolytic cleavage of myeloperoxidase does not require an acidic environment.