EFFECT OF ALBUMIN ON THE ESTIMATION, IN-VITRO, OF PHENYTOIN V-MAX ANDK-M VALUES - IMPLICATIONS FOR CLINICAL CORRELATION

The effect of bovine serum albumin (BSA) on human liver metabolism, in vitro, of C-14-phenytoin (PHT) was studied. Michaelis Menten paramete rs were determined for the conversion of PHT to p-hydroxy phenytoin in seven different microsomal preparations with the addition of 0, 2, an d 4% BSA. The unbound K-m (K-mu) values were 30.8 +/- 18.6, 1.57 +/- 0 .21 and 1.50 +/- 0.17 mu M (mean +/- S.D.), respectively; however, the re was excellent agreement among the V-max values (29.1, 31.8 and 31.5 pmol/min/mg). With intact tissue slices, BSA (4%) added to incubation s of PHT had a minimal effect on the V-max values in two of the four l ivers studied and resulted in a mean K-mu value of 2.20 +/- 0.59 mu M, although the K-mu in the absence of BSA was 6.64 +/- 3.17. In scaling -up to the whole body, V-max values were 3.9 and 1.0 mg/kg/day for mic rosomes and slices, respectively, compared to 5.9 mg/kg/day, in vivo. The K-mu values determined in the presence of albumin in both microsom es and slices were similar to those based on in vivo human steady stat e data (K-mu = 2-3 mu M), and the intersubject variation, in vitro, wa s decreased in the presence of BSA. These findings for phenytoin metab olism suggest that the addition of albumin to incubation media for sli ces or microsome experiments may yield K-m estimates that are more rep resentative of in vivo values.