Stereoselective binding of the enantiomers of four closely related N-methyl-barbiturates to human, bovine, and rat serum albumin

Albumin binding for the enantiomers of four closely related N-methyl-5-phenyl-5-alkyl-barbiturates 1-4 was investigated for three different mammalian species by means of equilibrium dialysis. Lipid solubility (n-heptane/phosphate buffer distribution coefficient) increased stepwise by a factor of 56 from 1 to 4. Bovine serum albumin: The (S)-(+)-enantiomers of 1-4 were bound in a higher percentage than the (R)-(-)-enantiomers; lengthening of the aliphatic side-chain increased the binding extent in both enantiomeric groups. Human serum albumin: Binding of (S)-(+)-1 and (S)-(+)-4 was higher than that of the (R)-(-)-enantiomers; with (S)-(+)-2 and (S)-(+)-3 it was much lower than that of the corresponding (R)-(-)-enantiomers. Lengthening of the aliphatic side chain increased the binding extent of the (S)-(+)- as well as of the (R)-(-)-enantiomers, but with two exceptions: 1. The (S)-(+)-1 binding exceeded that of the (S)-(+)-2 by a factor of nearly two. 2. The binding extent of (R)-(-)-4 was not further increased in comparison to (R)-(-)-3. Rat serum albumin: (S)-(+)-1 and (S)-(+)-2 were bound in a lower percentage than the (R)-(-)-enantiomers, both 3-enantiomers showing an equal binding extent; (S)-(+)-4 was bound to a slightly greater extent than the (R)-(-)-4. In the group of the (S)-(+)-enantiomers, the binding: extent increased from 1 to 4, whereas in that of the (R)-(-)-enantiomers only between 1 and 4. Structural differences between the serum albumins of three mammalian species possibly cause the enantioselective binding pattern found for the enantiomers of 1-4, and are responsible for the finding that the binding extent in some cases did not correlate with the lipid solubility of the compounds.