Saturable binding of [3H]phenytoin to rat brain membrane fraction

Preliminary studies indicate that [3H]phenytoin binds in a saturable and reversible fashion to at least two distinct sites in the membrane fraction of whole rat brain. One of these displays a high affinity (Kd = 6 nM) and a low maximal capacity (Bmax = 10 pmol/g protein). The other has a low affinity (Kd = 4.8 microM) and is estimated to have a very high maximal capacity. Phenytoin binding is reduced if the membrane fraction is preincubated with proteolytic enzymes and subcellular fractionation studies indicate that P2 fraction has the largest number of binding sites. Competition experiments fail to reveal significant binding interactions with putative neurotransmitters or with other drugs except the hydantoins and anticonvulsant barbiturates. Although it is premature to speculate on the clinical significance of these findings, it is encouraging to note that the low affinity site has a Kd very similar to the therapeutic levels of phenytoin found in cerebrospinal fluid and that there seems to be some relationship between binding potency and anticonvulsant potency within the hydantoin series.