sterigmatocystin + S-adenosyl-L-methionine <=> 8-O-methylsterigmatocystin + S-adenosyl-L-homocysteine + H(+)
sterigmatocystin + s-adenosyl-l-methionine <=> 8-o-methylsterigmatocystin + s-adenosyl-l-homocysteine + h(+)