AJP - Lung Cellular and Molecular Physiology, Vol 269, Issue 2 261-L271, Copyright © 1995 by American Physiological Society
gamma-Glutamyl transpeptidase is a polarized alveolar epithelial membrane protein
D. H. Ingbar, K. Hepler, R. Dowin, E. Jacobsen, J. M. Dunitz, L. Nici and J. D. Jamieson
Department of Medicine, University of Minnesota, Minneapolis 55455, USA.
In many diseases the lung is injured by oxidants. gamma-Glutamyl
transpeptidase (GGT) is an ectoenzyme on the apical plasma membrane of many
epithelial cells that protects against oxidants by replenishing
intracellular glutathione. We sought to localize GGT within rat lungs in
vivo and in cultured alveolar epithelial cells. In the adult rat lung,
indirect immunofluorescence (IF) with a polyclonal antibody to
triton-solubilized GGT revealed linear staining outlining the alveoli.
Immunoelectron microscopy (IEM) localized the protein on the apical surface
of the alveolar epithelial cells, but more densely on type I cells than
type II cells, as well as on the apical surface of some ciliated bronchial
cells. On Western blots of whole lung and isolated type II cell membrane
proteins, the antibody predominantly recognized a broad protein band of
110-120 kDa, consistent with the uncleaved, glycosylated form of GGT. Over
time in culture, isolated rat type II cells had increasing immunoreactivity
on Western blots and indirect IF but decreasing enzyme activity. At 2 days
in culture, confocal laser scanning microscopy demonstrated that GGT was
polarized to the apical surface of nonconfluent type II cells. Thus GGT is
a polarized apical membrane protein in type I and II cells, suggesting a
role in the metabolic functions of these cells. The increased
immunoreactive GGT of cultured type II cells is consistent with their
acquisition of properties similar to type I cells, but the lack of
correlation between immunoreactive protein and enzyme activity awaits
explanation.
