Presence of cytochrome b-558 in guinea-pig alveolar macrophages — subcellular localization and relationship with NADPH oxidase

Abstract

The assignment of cytochrome b-558 as a component of the O ⨪ 2 (H 2O 2) -generating enzyme in guinea-pig alveolar macrophages was investigated. Guinea pig alveolar macrophages contained 76 pmol cytochrome b-558 mg protein, a value very similar to that of neutrophils. The rate of myristic acid-stimulated O ⨪ 2 generation by alveolar macrophages, calculated per cytochrome b-558, was only one-fourth that of neutrophils. An analysis of Percoll density gradient centrifugation profiles showed that the H 2O 2-generating activity of myristic acid-activated alveolar macrophages was concentrated in a single peak which was consistently associated with 5′-nucleotidase activity, a plasma membrane marker enzyme. A little H 2O 2-generating activity was seen with unactivated alveolar macrophages. Furthermore, the cytochrome b-558 of both myristic acid-activated and unactivated alveolar macrophages was also predominantly associated with 5′-nucleotidase activity and was found in trace amounts in a peak containing lysozyme activity, a marker of lysosome granules. Only about 6% of the cytochrome b-558 in plasma membranes from myristic acid-activated guinea-pig alveolar macrophages was anaerobically reduced by 0.5 mM NADPH, while under the same conditions about 30% of the heme protein of myristic acid-activated neutrophils was reduced. These results suggest two conclusions: firstly, that in both activated and unactivated alveolar macrophages, cytochrome b-558 is located in the plasma membrane, and the translocation of cytochrome b-558 does not occur during the activation of NADPH oxidase; and secondly, that a smaller part of cytochrome b-558 is associated with the activated NADPH oxidase of guinea pig alveolar macrophages compared with neutrophils.