isorhamnetin + S-adenosyl-L-methionine => 3,3'-O-dimethylquercetin + S-adenosyl-L-homocysteine + H(+)
isorhamnetin + s-adenosyl-l-methionine => 3,3'-o-dimethylquercetin + s-adenosyl-l-homocysteine + h(+)