Synthesis of water-soluble bile pigments bound to amine-ended monomethoxypolyethyleneglycol: thiol addition and attempted enzymatic reduction of a bilindione derivative

Abstract.

The water-soluble amide to an NH₂-ended monomethoxypolyethyleneglycol (MPEG-NH₂, molecular mass of about 2000) of the dipyrrinone xanthobilirubic acid (XBR, 1) and the bis-amides of mesobiliverdin-XIIIα (MBV, 2) and mesobilirubin-XIIIα (MBR, 3) have been prepared with high yields. Contrary to what is observed with biliverdin-IXα, 4, the enzymatic reduction of the mesobiliverdin derivative 2-MPEGA to the corresponding mesobilirubin 3-MPEGA by the soluble biliverdin reductase/NADPH system in pH 7.4 aqueous phosphate does not occur. In contrast, thiol addition to 2-MPEGA and to 4 under similar conditions is immediate, although this equilibrium is slightly less favourable for 2-MPEGA. These results enable us to discount the intrinsically low reactivity of 2-MPEGA towards thiols as the reason for its lack of enzymatic reduction, and suggest instead that this particular mesobiliverdin cannot fit properly into the enzyme binding site, either because of steric hindrance or the lack of the two propionic acid groups.