Ectonucleotidases of avian gizzard smooth muscle and liver plasma membranes: A comparative study

Extracellular nucleotides, e.g., ATP, ADP, and UTP, are important signaling molecules which elicit various physiological responses in different tissue s. Their degradation is catalyzed by ectonucleotidases which are located on cell surfaces. Most tissues have a mixed population of ectonucleotidases. In this report, the ATP and ADP hydrolyzing ectonucleotidases of chicken gi zzard smooth muscle and liver plasma membranes were studied. The two membra nes exhibited marked differences in the ratio of ATPase/ADPase activities, activation by divalent cations, thermal stability, responses to detergents and cross-linking agents, and sensitivity to several enzyme inhibitors. The ATPase activity of chicken gizzard membranes is (i) labile to heat and det ergents; (ii) activated by concanavalin A and disuccinimidyl suberate, both crosslinking agents; (iii) inhibited by mercurials; and (iv) insensitive t o high concentrations of azide, a known inhibitor of ecto-ATP diphosphohydr olases (ecto-ATP/ Dase). In contrast, the liver membrane ATPase and ADPase activities are more stable to treatment by heat and detergents and insensit ive to cross-linking agents and mercurials, but are inhibited by azide, A l ow ADP hydrolase activity in the gizzard membranes could be distinguished f rom both the gizzard ATPase and the liver ATPase/ADPase. This ADP hydrolase , which is markedly stimulated by NBD-CI, accounts for most of the ADP hydr olysis activity in gizzard membranes. It is concluded that the major ectonu cleotidase in the gizzard membranes is an ecto-ATPase whereas that in the l iver membranes is an ecto-ATP/Dase. That both membranes contain a mixed pop ulation of the ecto-ATPase and ecto-ATP/Dase, but in different proportions, is further demonstrated by immunochemical characterization. The different composition of ectonucleotidases in the two membranes is expected to have a n important effect on the regulation of hydrolysis of extracellular ATP as well as the concentration of extracellular adenine nucleotides in the gizza rd and liver tissues. (C) 1999 Academic Press.