Localization of Phosphatidic Acid Phosphatase Activity in Granular Pneumonocytes

The alveoli of the mammalian lung are lined by a film of surface-active material (surfactant) which is commonly believed to be a phospholipid (1, 5, 8, 11) . A considerable body of evidence has accumulated which suggests that surfactant is synthesized by the granular pneumonocytes (type II cells) of the alveolar epithelium (1, 9, 10) . Although a number of acid hydrolases have been demonstrated in the inclusion bodies of granular pneumonocytes (4, 6), no attempt has been made to localize enzymes specifically concerned with phospholipid metabolism. Phosphatidic acid phosphatase is of particular interest because of its key role in the biosynthesis of phosphatides . Unfortunately, this enzyme cannot be demonstrated by conventional histochemical methods because of the insolubility of the substrate (phosphatidic acid) in water . Stable emulsions of phosphatidic acid can be prepared but these do not diffuse into cells . One method of overcoming this difficulty is to inflict minor localized damage on cells and expose the organelles to an emulsion of phosphatidic acid in a suitable medium . A method of producing minor trauma by osmotic forces has been developed to enable the localiza-