Provided herein are methods for treating a subject suffering from phenylketonuria by administering a phenylalanine hydroxylase (“PAH”) and/or a phenylalanine ammonia lyase (“PAL”) to the subject under conditions effective to deliver the phenylalanine 4-hydroxylase and/or PAL to the subjects small intestine. Also provided are methods for increasing the therapeutic activity of a phenylalanine 4-hydroxylase by thiolating the phenylalanine 4-hydroxylase. In addition, provided are oral dosage forms that include a phenylalanine 4-hydroxylase and/or a PAL and an enteric coating.
