Multiple species of myeloperoxidase messenger RNAs produced by alternative splicing and differential polyadenylation

Preferential adsorption of human neutrophil myeloperoxidase isoform III by oral bacteria

Neutrophil myeloperoxidase (MPO) adsorbs to bacteria as a pre-requisite for killing by the MPO/hydrogen-peroxide/chloride system. Three chromatographically distinct isoforms of MPO (MPO I, MPO II, and MPO III) have been isolated from human neutrophils. The purpose of this study was to determine whether oral bacteria--including Actinobacillus actinomycetemcomitans, Capnocytophaga sputigena, Haemophilus aphrophilus, and Eikenella corrodens--differentially adsorb the three major isoforms of MPO from a mixture of MPO I, II, and III, and to assess the effect of pH and normal human serum (NHS) on MPO adsorption. MPO III adsorbed preferentially (i) at high bacterial concentrations, (ii) in the pH range of 6.0-8.0, and (iii) in the presence and absence of NHS. These results support the role of MPO III in the killing of oral bacteria.

Properties of human liver cytosolic aspartate aminotransferase mRNAs generated by alternative polyadenylation site selection

Human cytosolic aspartate aminotransferase (cAspAT) cDNA clones have been isolated from an adult human liver cDNA library. Among the clones, two cDNAs of 1550 and 1950 base pairs, respectively, have been characterized. These two cDNAs differ only in the lengths of their 3' noncoding regions and by the presence of one or two putative polyadenylation signals AATAAA. Northern blot analysis revealed two different mRNAs of 2.1 and 1.8 kbp in several human tissues, whereas Southern blot analysis suggested the existence of a single gene for the human cAspAT. The two mRNA species result from the alternative use of two polyadenylation signals. In the liver, the relative ratio of these mRNAs varies among different species and, in humans at least, during development. The properties of the two mRNAs were compared. The half-lives of the 2.1 and 1.8 kbp mRNAs, in the HepG2 cell line, are 8 and 12 h, respectively. The two mRNAs have similar and rather short poly(A) tracts of 20-50 nucleotides. Both mRNAs are capable of directing the in vitro synthesis of the cAspAT protein. We conclude that both the 2.1 and 1.8 kbp cAspAT mRNAs are functional and exhibit similar properties.

Assembly of dimeric myeloperoxidase during posttranslational maturation in human leukemic HL-60 cells

Myeloperoxidase is a major protein component of the azurophilic granules (specialized lysosomes) of normal human neutrophils and serves as part of a potent bactericidal system in the host defense function of these cells. In normal, mature cells, myeloperoxidase occurs exclusively as a dimer of Mr 150,000 while in immature leukemia cells, there are both monomeric (Mr 80,000) as well as dimeric species. Like other lysosomal enzymes, myeloperoxidase is synthesized as a larger glycosylated precursor (Mr 91,000) that undergoes processing through single-chain intermediates (Mr 81,000 and 74,000) to yield mature heavy (Mr 60,000) and light (Mr 15,000) subunits. To study the assembly of dimeric myeloperoxidase, azurophilic granules were isolated from either unlabeled or pulse-labeled ([35S]methionine/cysteine) HL-60 cells, and myeloperoxidase was extracted and separated into monomeric and dimeric forms by FPLC gel filtration chromatography. Steady-state levels of dimeric and monomeric myeloperoxidase were found to account for 67% and 33%, respectively, of the total peroxidase activity and were correlated with the levels of associated heme as measured by absorption at 430 nm. Labeled myeloperoxidase polypeptides were immunoprecipitated using a monospecific rabbit antibody and were identified and quantitated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis/fluorography and liquid scintillation counting. After a 2-h pulse, labeled myeloperoxidase species of Mr 74,000 and 60,000 were found in fractions coeluting with the monomeric form of myeloperoxidase.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and characterization of extracellular myeloperoxidase precursor in HL-60 cell cultures

An extracellular myeloperoxidase precursor of HL-60 cells was purified from the culture supernatant by ammonium sulfate precipitation, DEAE-Sepharose chromatography, and monoclonal antibody affinity chromatography. The purified protein was a glycoprotein of approximately 89 kDa as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The amino-terminal amino acid sequence of the protein began at amino acid residue 49 of the 745-amino acid sequence deduced from a myeloperoxidase cDNA, suggesting that the protein consisted of 697 amino acid residues. The implications of the precursor in the processing of myeloperoxidase are discussed.