Study of deutero-isotopomer-induced inhibition of caffeine and phenobarbitone binding to human serum albumin.

The present study of inhibition provides confirmation to previously observed deuterium isotope effects on in vitro caffeine and phenobarbitone binding to human serum albumin (HSA). Addition of either 3,7(C(2H)3)2 or 1,3,7(C(2H)3)3 caffeine induces a 50% loss in both the extent of binding and binding parameters of the unlabelled analog, understandably so in view of the stronger individual HSA binding of the two labelled isotopomers. As concerns caffeine displacement from its HSA sites, we show phenobarbitone and its 5-pentadeuterophenyl analog are equally potent inhibitors of caffeine binding, though their individual HSA binding profiles differ. As for HSA binding interactions between phenobarbitone isotopomers, a 50% decrease in unlabelled phenobarbitone extent of binding is observed in the presence of its 5-pentadeuterophenyl analog. Our results favor the hypothesis of differing binding sites for each isotopomer.