title: Effects of aluminum on bovine parathyroid adenylate cyclase.
abstract: It is known that the secretion of PTH is often impaired in association with aluminum (Al3+) accumulation in patients with renal failure. The mechanisms involved remain ill defined. Since adenylate cyclase plays a role in the regulation of PTH secretion, these studies examine the effects of Al3+ on parathyroid adenylate cyclase. In membranes from normal bovine parathyroid glands, basal adenylate cyclase activity, in the presence of 0.2 mM ATP and 20 mM Mg2+, increased by 22% as Al3+ was raised from 0-10 microM. Higher Al3+ concentrations caused a progressive decrease in adenylate cyclase activity, reaching 68% inhibition of control activity at 2 mM Al3+. Since adenylate cyclase activation is influenced by the interaction of multiple sites within the adenylate cyclase complex, the nature of the inhibition by Al3+ was explored by examining the interaction of Al3+ with substrate ATP and with Mg2+, an allosteric activating metal ion. In the presence of 20 mM Mg2+, Al3+ concentrations of 1-2 mM resulted in noncompetitive inhibition with respect to ATP [decrease in maximum velocity (Vmax) from 4176 in the absence of Al3+ to 1106 pmol cAMP/mg protein X 15 min; Michaelis Menten constant (Km) for ATP was unchanged]. In contrast, at fixed ATP (0.2 mM), 0.5 mM resulted in competitive inhibition of adenylate cyclase with respect to Mg2+, whereas at higher Al3+ concentrations the inhibition was noncompetitive. When Mg2+ was replaced by Mn2+ (enzyme activity reflects the activity of the catalytic unit), the inhibitory effect of Al3+ on adenylate cyclase activity was abolished. These data suggest that the inhibition of parathyroid adenylate cyclase by Al3+ occurs at the level of the allosteric metal activating site. These data provide a potential mechanism for the inhibition of PTH secretion by Al3+.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000262:Adenylyl Cyclases / Q000378:metabolism*; D000535:Aluminum / Q000494:pharmacology*; D000077410:Aluminum Chloride; D017607:Aluminum Compounds; D000818:Animals; D002122:Calcium Chloride / Q000494:pharmacology; D002417:Cattle; D002712:Chlorides; D006160:Guanosine Triphosphate / Q000378:metabolism; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D008345:Manganese / Q000378:metabolism; D010280:Parathyroid Glands / Q000187:drug effects / Q000201:enzymology*
chemical_list: D017607:Aluminum Compounds; D002712:Chlorides; D000077410:Aluminum Chloride; D008345:Manganese; D006160:Guanosine Triphosphate; D000255:Adenosine Triphosphate; D000535:Aluminum; D000262:Adenylyl Cyclases; D008274:Magnesium; D002122:Calcium Chloride