Molecular mechanism of the lead-induced inhibition of rod cGMP phosphodiesterase.
Retinal cGMP hydrolysis is inhibited following developmental and in vitro lead exposure. To determine whether Pb2+ directly inhibits the rod-specific cGMP phosphodiesterase (PDE) and to examine the kinetic mechanism of this inhibition, purified bovine rod cGMP PDE was assayed at varying [cGMP], [Mg2+] and [Pb2+]. Increasing [cGMP] or [Mg2+] shifted the Pb2+ curves leftward or rightward revealing increased or decreased potency of Pb2+ (nM to pM range), respectively. A 1/velocity vs. 1/mg2+ plot revealed that picomolar [Pb2+] competitively inhibited PDE relative to millimolar [Mg2+]. These novel findings reveal that Pb2+ binds at the Mg2+ site but with 4-6 log units higher affinity, thus preventing cGMP hydrolysis. These results may have implications for other enzymes using Mg2+ as a co-factor and suggest that Mg2+ may be useful for reversing the PDE inhibition by Pb2+.
Fox
D A
DA
College of Optometry, University of Houston, TX 77204-6052, USA. DAFox@uh.edu
Srivastava
D
D
D015106:3',5'-Cyclic-GMP Phosphodiesterases / Q000037:antagonists & inhibitors*; D000818:Animals; D002417:Cattle; D006152:Cyclic GMP / Q000378:metabolism; D007854:Lead / Q000633:toxicity*; D008274:Magnesium / Q000494:pharmacology; D010786:Photoreceptor Cells / Q000187:drug effects* / Q000201:enzymology; D051381:Rats
D007854:Lead; D015106:3',5'-Cyclic-GMP Phosphodiesterases; D006152:Cyclic GMP; D008274:Magnesium