S-Adenosyl-L-homocysteine hydrolase plays a critical role in regulating cellular methylation reactions by catalyzing the breakdown of S-adenosyl-L-homocysteine (SAH) into adenosine and homocysteine. This reaction is essential because SAH acts as a potent feedback inhibitor of methyltransferases, enzymes responsible for transferring methyl groups in processes such as DNA, RNA, protein, and phospholipid methylation. By removing SAH, the hydrolase ensures that methylation reactions can proceed efficiently, maintaining proper epigenetic regulation and gene expression.

In biomedical research, this enzyme is a target for drug development, particularly in antiviral and anticancer therapies. Inhibitors of S-adenosyl-L-homocysteine hydrolase lead to accumulation of SAH, which in turn suppresses viral replication by interfering with viral mRNA cap methylation. This approach has been explored for treating infections caused by RNA viruses such as hepatitis C, influenza, and HIV.

Additionally, dysregulation of this enzyme is linked to metabolic and neurological disorders due to altered methylation capacity. Modulating its activity offers a pathway to study and potentially treat conditions involving abnormal methylation patterns, including liver disease, depression, and neurodegenerative diseases.