摘要：

The liver parenchymal cell plasma membrane preparation devised by Seville (Biochem. Biophyr. A&, 154, 540 (1968)) contains an adenyl cyclase system which ls stimulated by glucagon and to a much lesser extent by epinephrine. The yield of adenyl cyclase can be greatly increased, at the expense of some contamination by other organelles, by lncreaslng the amount of starting material and eliminating the last step in the preparative procedure. Adenyl cyclase activity is 25-fold purified ln the membranes compared to a liver homogenate. Liver membrane adenyl cyclase activity is a function of glucagon concentration over a range of lo+ to lo-' y glucagon and ls increased more than lo-fold by a maximally stimulating concentration of glucagon. The stimulation produced by epinephrlne is less than 10% of that produced by glucagon. Secretin, a polypeptide hormone very similar to glucagon ln its primary structure, does not stimulate liver membrane adenyl cyclase activity. Insulin changes neither the basal nor the glucagon-stimulated adenyl cyclase activity. Adenyl cyclase activities, under standard assay conditions, are proportional to time of incubation and to membrane concentration. The enzyme requires a divalent cation, either Mg++ or Mn++, but is inhibited by Ca++. At sufficiently high concentrations, both ATP and Mg++ inhibit the enzyme. Addition of 1 my EDTA to the assay medium doubles the glucagon-stimulated activity. Fluoride ion stimulates adenyl cyclase activity but to a lesser extent than does glucagon. Glutaraldehyde, N-ethylmalelmlde, and p-chloromercuribenzoate inhibit the adenyl cyclase system. Urea, at a concentration of 2 Y, reduces both the maximal glucagonand fluoride-stimulated activities, but at 0.4 M reduces only the apparent a5nity of the system for glucagon. Attempts to solubilize and purify the adenyl cyclase activity ln an active form from rat liver membranes have been unsuccessful.

展开