(S)-Mephenytoin is an anticonvulsive drug which is metabolized by N-demethylation and 4-hydroxylation (of the aromatic ring). (S)-Mephenytoin is a substrate of the cytochrome P450 (CYP) isoform CYP2C19, also known as mephenytoin 4-hydroxylase [1].

CYP2C19 is the main mephenytoin 4-hydroxylase in humans implicated in metabolizing a variety of therapeutic agents, such as omeprazole, proguanil, diazepam, propranolol, citalopram, imipramine, and certain barbiturates [2].

In vitro: In the presence of cytochrome b5, the Km value for S-mephenytoin was 1.25 mM with all five purified cytochrome P-450s preparations, and Vmax values were 0.8-1.25 nmol of 4-hydroxy product formed per min/nmol of P-450 [1].

References:
[1] Shimada T, Misono K S, Guengerich F P.  Human liver microsomal cytochrome P-450 mephenytoin 4-hydroxylase, a prototype of genetic polymorphism in oxidative drug metabolism. Purification and characterization of two similar forms involved in the reaction[J]. Journal of Biological Chemistry, 1986, 261(2): 909-921.
[2] Ferguson R J, De Morais S M F, Benhamou S, et al.  A new genetic defect in human CYP2C19: mutation of the initiation codon is responsible for poor metabolism of S-mephenytoin[J]. Journal of Pharmacology and Experimental Therapeutics, 1998, 284(1): 356-361.