In enzymology, a putrescine N-methyltransferase is an enzyme that catalyzes the chemical reaction Thus, the two substrates of this enzyme are S-adenosyl methionine and putrescine, whereas its two products are S-adenosylhomocysteine and N-methylputrescine. This enzyme belongs to the family of transferases, specifically those transferring one-carbon group methyltransferases. The systematic name of this enzyme class is S-adenosyl-L-methionine:putrescine N-methyltransferase. This enzyme is also called putrescine methyltransferase. This enzyme participates in alkaloid biosynthesis ii. This enzyme is important in the synthesis of many plant alkaloids. It evolved from spermidine synthase.