Molecular Pharmacology, Vol 13, 172-180, Copyright © 1977 by the American Society for Pharmacology and Experimental Therapeutics

Effects of Sodium Chloride on Phenylethanolamine N-Methyltransferase Activity

¹ Department of Pharmacology and Toxicology, Faculty of Pharmacy, Universidad Central de Venezuela, Caracas, Venezuela

NaCl and KCl at the concentrations present in body fluids competitively inhibited phenylethanolamine N-methyltransferase activity whether octopamine, normetanephrine, or norepinephrine was used as substrate. This inhibitory effect was more pronounced at low concentrations of S-adenosylmethionine. Both salts reduced the inhibition of enzyme activity observed at high N-methyl acceptor concentrations. This salt effect was dependent on the buffer employed, the pH of the assay, and the S-adenosylmethionine concentration. Similar effects were produced by equal ionic strength solutions of NaF, NaBr, CsCl, LiCl, K₂SO₄, Na₂SO₄, MgSO₄, and CaCl₂. In addition, an increase in the ionic strength of the buffer (Tris-HCl or phosphate) reduced the enzyme activity at subsaturating amine concentrations but enhanced the reaction velocity at high substrate concentrations. The optimum pH varied, depending on the amine concentration employed. The present results suggest that an increase in the ionic strength of the salt solution might reduce binding of substrate molecules to the enzyme, which would be manifested by a decrease in the reaction velocity at low amine concentrations and by a reduction of the inhibitory effect produced by saturating amine concentrations.

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ACKNOWLEDGMENT The authors are grateful to Dr. Richard Deitrich for his advice in the preparation of the manuscript.