Lanthanide ions inhibit the activity of dihydrofolate reductase from chicken liver.
The influences of mono-, bi- and trivalent metal ions (as chloride salts) on the activity of dihydrofolate reductase (DHFR) from chicken liver have been studied to elucidate the mechanism of ion-activation of this enzyme. The results show that monovalent ions (Na+ and K+) activate DHFR at low concentration reaching a maximum activation of about 2.5 fold at 0.4-0.5 M and declining at higher concentrations. Ca2+ shows similar activation but at lower concentration, reaching a maximum at 0.1 M; activity declines with further increases in concentration. At very high concentration (> 0.4 M), Ca2+ is inhibitory. The trivalent lanthanide ions, however, show a dramatic inhibition of activity of DHFR even at very low concentration. The activity of DHFR declines to 50% of that of the control at 0.02 mM EuCl3. Intrinsic fluorescence measurements show that the ion-dependent activation in the presence of mono- and bivalent metal ions is due to the conformational changes in the protein. Energy transfer phenomenon suggests that the specific interaction of Eu3+ with Trp24 located in a loop at the active site of DHFR is responsible for the strong inhibition. The possible mechanism for the ion-inhibition is proposed and discussed.
Wu
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Department of Chemistry, Penn State University, University Park, PA 16802, USA. yxw19@psu.edu
D000818:Animals; D002413:Cations, Divalent; D002414:Cations, Monovalent; D002645:Chickens; D004735:Energy Transfer / Q000187:drug effects; D004789:Enzyme Activation / Q000187:drug effects; D005063:Europium / Q000378:metabolism / Q000494:pharmacology; D005493:Folic Acid Antagonists / Q000378:metabolism / Q000494:pharmacology*; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D008163:Luminescent Measurements; D008674:Metals, Rare Earth / Q000378:metabolism / Q000494:pharmacology*; D011189:Potassium Chloride / Q000378:metabolism / Q000494:pharmacology; D012965:Sodium Chloride / Q000378:metabolism / Q000494:pharmacology; D013050:Spectrometry, Fluorescence; D013762:Tetrahydrofolate Dehydrogenase / Q000378:metabolism*
D002413:Cations, Divalent; D002414:Cations, Monovalent; D005493:Folic Acid Antagonists; D008674:Metals, Rare Earth; D005063:Europium; D012965:Sodium Chloride; D011189:Potassium Chloride; D013762:Tetrahydrofolate Dehydrogenase