Molecular Pharmacology, Vol 19, 463-469, Copyright © 1981 by the American Society for Pharmacology and Experimental Therapeutics

Interaction of S-Adenosylhomocysteine with Isolated Rat Hepatocytes

¹ Department of Pharmacology, University of Bergen School of Medicine, Bergen, Norway

The time course of the association of radioactivity with isolated hepatocytes incubated in the presence of S-[¹⁴C]adenosylhomocysteine was characterized by an initial burst phase lasting for less than 15 sec (Phase I) followed by a slow linear phase (Phase II). Phase I was due to association of S-[¹⁴C]adenosylhomocysteine with the hepatocytes. The possibiity of intracellular resynthesis of S-[¹⁴C]adenosylhomocysteine from [¹⁴C]adenosine was ruled out. The process showed slight temperature dependence, and there was no effect of 2,4-dinitrophenol. These data argue against the possibility that Phase I was related to active uptake of S-[¹⁴C]adenosylhomocysteine, but rather suggest binding of this compound to an acceptor on the cell surface. Competition studies showed that S-adenosylhomocysteine was the preferred ligand of this acceptor. Phase II corresponded to intracellular accumulation of ¹⁴C-labeled adenine nucleotides and was nearly completely abolished by inhibitors of enzymatic hydrolysis of S-adenosylhomocysteine, such as adenosine and homocysteine. This process was explained by extracellular hydrolysis of S-[¹⁴C]adenosylhomocysteine catalyzed by S-adenosylhomocysteine hydrolase leaking out of the cells, followed by rapid uptake of [¹⁴C]adenosine. Trapping of adenosine formed from S-adenosylhomocysteine directed the extracellular enzyme catalysis towards hydrolysis. Extracellular hydrolysis of S-adenosylhomocysteine was the rate-limiting step in this sequence of events. It is concluded that S-adenosylhomocysteine was not taken up by rat hepatocytes as an intact molecule.

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ACKNOWLEDGMENTS The authors wish to thank Hallvard Bergesen and Audun Høylandskær for excellent technical assistance.