Biological effects of inhibitors of S-adenosylhomocysteine hydrolase.

S-Adenosylhomocysteine (AdoHcy), formed after the donation of the methyl group of S-adenosylmethionine to a methyl acceptor, is hydrolyzed to adenosine and homocysteine by AdoHcy hydrolase physiologically. The administration of the inhibitors of AdoHcy hydrolase to cells or animals normally results in an accumulation of cellular AdoHcy higher than those found in controls, which is often accompanied by a simultaneous rise in S-adenosylmethionine because of the feedback inhibition by AdoHcy on most methylation reactions. AdoHcy hydrolase has become a tantalizing pharmacological target for inhibition since its blockade can affect cellular methylation of phospholipids, proteins, small molecules, DNA, and RNA. Indeed, all of these different methylation reactions have been found to be inhibitable by the nucleoside inhibitors/substrates of AdoHcy hydrolase. Among the interesting effects are the activation of genes, induction of cellular differentiation, increased expression of transcription factors, and sometimes the repression of genes. Furthermore, some of the nucleosides show remarkable antiviral activities in vitro and in vivo. However, the mode of action of the inhibitors appears complex. Although the inhibition of methylation might account for some of the biological effects, the ability of some of the nucleoside inhibitors to undergo metabolic phosphorylation to nucleotides may account for part of their biological activities. The defining mode of action responsible for their biological effects still awaits biochemical elaboration, especially regarding their antiviral effects, induction of genes, or cellular differentiation.