Presence of adenylate cyclase activity in Golgi and other fractions from rat liver. I. Biochemical determination

The distribution of adenylate cyclase (AC) in Golgi and other cell fractions from rat liver was studied using the Golgi isolation procedure of Ehrenreich et al. In liver homogenate the AC activity was found to decay with time, but addition of 1 mM EGTA reduced the rate of enzyme loss. The incorporation of 1 mM EGTA into the sucrose medium used in the initial two centrifugal steps of the Golgi isolation method stabilized the enzyme activity throughout the entire procedure and resulted in good enzyme recovery. In such preparations, AC activity was demonstrated to be associated not only with plasma membranes but also with Golgi membranes and smooth microsomal membranes as well. Furthermore, under the conditions used, enzyme activity was also associated with the 105,000 g x 90 min supernatant fraction. The specific activity of the liver homogenate was found to be 2.9 pmol-mg protein-1-min-1, the nonsedimentabel and microsomal activity was of the same order of magnitude, but the Golgi and plasma membrane activities were much higher. The specific activity of plasma membrane AC was 29 pmol-mg proten-1-min-1. The Golgi activity varied in the three fractions, with the highest activity (14 pmol) in GF1 lowest activity (1.8) in GF2, and intermediate activity (5.5) in GF3, when the Golgi activity was corrected for the presence of content protein, the activity in GF1 became much higher (9 x) than that of the plasma membrane while the activities in GF2 and GF3 were comparable to that of plasma membrane. In all locations studied, the AC was sensitive to NaF stimulation, especially the enzyme associated with Golgi membranes. The activities in plasma and microsomal membranes were stimulated by glucagon, whereas the Golgi and nonsedimentable AC were not.