Table 2.

Association equilibrium constants (K_a) and types of binding for phenytoin and its metabolites to various regions on HSA^a

Binding region on HSA	Drug or drug metabolite
	Phenytoin	m-HPPH	p-HPPH
Sudlow site Ib	Allosteric effects + possible direct binding	No binding	No binding
Sudlow site IIb	Direct binding Ka = 1.04 × 103 M−1	Direct binding Ka = 3.2 × 103 M−1	Direct binding Ka = 5.7 × 103 M−1
Digitoxin site	Direct binding Ka = 6.5 × 103 M−1	No binding	No binding
Tamoxifen site	Allosteric effects + possible Direct binding	No binding	No binding

^aAll of these results were obtained at 37ºC in pH 7.4, 0.067 M phosphate buffer and are based on data from Refs. [81,86].

^bSudlow sites I and II are also known as the warfarin site and indole site, respectively.