Cholera toxin activation of adenylate cyclase in cancer cell membrane fragments

Activation of adenylate [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] by cholera toxin (84,000 daltons, 5.5 S) is demonstrated in plasma membrane fragments of mouse ascites cancer cells. The activation of adenylate cyclase is mediated by a macromolecular cyclase activating factor (MCAF), which has a sedimentation constant of 2.7 S and a molecular weight of about 26,000. MCAF is derived from, and may be identical to the "A fragment" of cholera toxin. Generation of MCAF depends on prior interaction of cholera toxin with either dithiothreitol, NADH, NAD, or a low-molecular-weight component (less than 700 daltons) present in cytoplasm. Subsequent exposure of this pretreated cholera toxin to cell membranes from a variety of mouse ascites cancer cells is followed rapidly by the appearance of MCAF, which no longer requires dithiothreitol, NADH, or NAD for the activation of adenylate cyclase. Activation of adenylate cyclase by MCAF in ascites cancer cell membrane fragments is not reversed by repeated washing of these membrane fragments. Adenylate cyclase in normal cell membrane fragments fails to respond either to cholera toxin or MCAF in the presence of dithiothreitol. In striking contrast, the adenylate cyclase in membrane fragments from five ascites cancer cells responds to either MCAF or native cholera toxin preincubated with dithiothreitol, NADH, or NAD.