This study characterizes recombinant human Protein arginine methyltransferase 1 (PRMT1) expressed in Escherichia. coli and reveals that the enzyme exhibits leaky expression even without isopropyl β-D-1-thiogalactopyranoside (IPTG) induction. Through in vitro histone methyltransferase assays, we demonstrate that PRMT1 activity toward histone H4R3 is enhanced by dithiothreitol (DTT) in a dose-dependent manner but remains unresponsive to β-mercaptoethanol. These findings highlight the critical role of redox conditions in regulating PRMT1 catalytic function and provide a practical framework for future biochemical and kinetic studies.