Inhibition of transducin activation and guanosine triphosphatase activity by aluminum ion.
Aluminum ion perturbs the activity of a number of physiologically important enzymes, including members of a family of guanine nucleotide-binding proteins (G-proteins). G-proteins couple cellular receptor proteins to a variety of effector enzymes (including adenylate cyclase, phospholipase C, and the rod photoreceptor phosphodiesterase). We show herein that subnanomolar concentrations of free aluminum ion, produced in a carefully defined and kinetically stable manner through the buffering of total aluminum at 0.1-1.0 mM with calculated ratios of chelating agents, inhibit both the receptor-mediated activation and the self-inactivating GTPase activity of the rod photoreceptor G-protein, Gv. In the presence of 4 X 10(-10) M free aluminum ion, GTPase activity is inhibited from about 25-60% as the magnesium ion concentration is reduced from 10(-3) to about 5 X 10(-5) M. The principal effect of aluminum ion upon Gv is to inhibit receptor catalyzed nucleotide exchange. Binding of the GTP analog 5'-guanylyl imidodiphosphate can be reduced by as much as 90% by aluminum ion following subsaturating rhodopsin stimulation. Aluminum ion can produce either competitive or mixed noncompetitive inhibition of rhodopsin-catalyzed Gv activation and GTPase activity, as a function of whether Gv undergoes single (competitive), or multiple (mixed noncompetitive) nucleotide exchanges. The rod photoreceptor phosphodiesterase is only slightly inhibited by similar aluminum ion activities. Light- and Gv-coupled phosphodiesterase activation exhibits both a lower maximum rate of cyclic guanosine monophosphate hydrolysis and a slower inactivation in the presence of aluminum ion activities from about 10(-12) - 10(-10) M. These data suggest that intracellular free aluminum ion concentrations in the subnanomolar range could markedly affect the ability of cells to transduce extracellular signals. Interestingly, the combination of Al3+ and F- to produce the fluoro-aluminate species (AlFx) also inhibits the GTPase of G-proteins, although the mechanism of inhibition (e.g. binding to the G-protein.Mg2+.GDP complex) is totally distinct from that observed for free Al3+ and the overall effect on signal transduction (e.g. enhanced signal amplification) is in complete opposition to that observed for free Al3+.
Miller
J L
JL
Department of Biochemistry, School of Medicine, University of Virginia, Charlottesville 22901.
Hubbard
C M
CM
Litman
B J
BJ
Macdonald
T L
TL
D015106:3',5'-Cyclic-GMP Phosphodiesterases / Q000037:antagonists & inhibitors; D000535:Aluminum / Q000494:pharmacology*; D000818:Animals; D002417:Cattle; D020558:GTP Phosphohydrolases / Q000037:antagonists & inhibitors*; D019204:GTP-Binding Proteins / Q000037:antagonists & inhibitors*; D006165:Guanylyl Imidodiphosphate / Q000378:metabolism; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D010744:Phosphoric Monoester Hydrolases / Q000037:antagonists & inhibitors*; D011485:Protein Binding; D012243:Rhodopsin / Q000378:metabolism; D012374:Rod Cell Outer Segment / Q000378:metabolism; D015293:Transducin / Q000037:antagonists & inhibitors*
D006165:Guanylyl Imidodiphosphate; D012243:Rhodopsin; D000535:Aluminum; D010744:Phosphoric Monoester Hydrolases; D015106:3',5'-Cyclic-GMP Phosphodiesterases; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins; D015293:Transducin; D008274:Magnesium