The Post-translational Processing of Myeloperoxidase Is Regulated by the Availability of Heme

Myeloperoxidase (MPO) is a hemoprotein that is synthesized in the lumen of the endoplasmic reticulum (ER) as a single-chain precursor and undergoes a complex series of post-translational modifications prior to packaging into azurophilic granules. We and others have previously observed that treatment of human myeloid leukemic cells with succinylacetone (SA), a potent inhibitor of 5-aminolevulinic acid dehydratase (ALA-D), and hence of heme biosynthesis, resulted in loss of MPO enzyme activity, inhibition of the appearance of mature MPO, and accumicrolation of enzymatically unreactive, but immicronoreactive, MPO in the ER. The present study using HL-60 cells was undertaken to establish the nature and specificity of the inhibition by SA and to identify and quantify the biochemical changes in the post-translational pathway of MPO processing. Dose-response studies showed that SA (250 μM) did not affect cell viability or growth up to 72 h, but resulted in inhibition of ALA-D activity (>93%) and decreased cellular levels of both heme and MPO (∼25% of control). There were no effects on the level of total cellular protein or on the activities of lactate dehydrogenase or several other nonheme enzymes colocalized with MPO in azurophilic granules. Northern blot analyses confirmed the nontoxic nature of the conditions and indicated there was no effect on transcription of MPO mRNA. The kinetics of processing in the presence and absence of 250 μM SA were determined using pulse-chase and Percoll density gradient centrifugation methods, followed by identification and quantification of MPO species by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. The initial rate of disappearance of precursor MPO was identical for control and SA-treated cells and, after a lag of 2-3 h, there was a fourfold decrease in the rate of appearance of mature MPO in SA-treated cells. In the presence of SA, precursor apoMPO remained in the ER, did not undergo proteolytic processing and, compared to control cells, about 50% was degraded. The disruption in MPO processing was reversible by the addition of exogenous heme. We conclude that the availability of heme is important in the complex maturation of MPO that occurs in the ER, events which precede exit from this compartment and subsequent proteolytic processing and transport to the azurophilic granule.