purified hypomethylated recombinant proteins Flag-KAP1 or, as negative control, Flag-ZNF224,
which is not methylated by PRMT5 [11]. To generate hypomethylated protein extracts that are good
in vitro substrates for methyltransferases, Flag-KAP1 and Flag-ZNF224 were purified from HEK293
cells cultured in the presence of adenosine periodate oxidized (AdOx) [14], an adenosylhomocysteine
hydrolase inhibitor that causes the accumulation of intracellular adenosylhomocysteine (AdoHcy)
levels. This increase in AdoHcy levels results in feedback inhibition of most methylation reactions.
Indeed, a number of specific PRMT inhibitors recently identified have proven to be of limited use due
to their inability to enter cells or their cytotoxic effects [15].