Immunohistochemical localization and characterization of a rat Clara cell 26-kDa protein (CC26) with similarities to glutathione peroxidase and phospholipase A₂

We have purified and partially sequenced a 26-kDa protein isolated from rat lung lavage. Two-dimensional electrophoresis and Western blotting using an antibody we have raised to this protein indicated that CC26 has 3 isoforms with pIs between 4.9 and 5.5 and is neither a component of surfactant nor present in plasma. The first 10 amino acids of the N-terminal of all 3 isoforms were identical. The first 25 amino acids of the N-terminal sequence were identical to a rat acidic calcium-independent phospholipase A'2 and one amino acid different from a mouse nonselenium glutathione peroxidase. Light immunohistochemistry showed a strong reaction with the airway hypophase from the trachea down to the terminal bronchioles, but not in the alveolus. Immunohistochemistry at the electron microscopy level showed that CC26 was localized to the dense secretory bodies and endoplasmic reticulum of the Clara cell and secretory granules of tracheal nonciliated serous and goblet cells. Heavily labeled Clara cell dense secretory bodies were observed in the process of being exocytosed. Biochemical and further sequence analysis will be required to determine if this protein is either a nonselenium glutathione peroxidase or a calcium-independent phospholipase A₂.