The degree of human serum albumin (HSA) binding for 16 different homopyrimidazole derivatives has been studied. The positions of UV absorption maximum, the corresponding molar extinction coefficients and the binding parameters obtained by equilibrium dialysis experiments at 4 degrees C, in pH 7.35 phosphate buffer, are surveyed. In most cases compounds with high toxicity show low binding tendency. Saturation of the ring system decreases binding, while an increase of the number of double bonds enhances the binding affinity. Important requirements for HSA binding of homopyrimidazole derivatives are the presence of a carbethoxy (or carboxyl) group in position 3 and a C6-methyl group. The highest binding affinity could be demonstrated in the case of MZ 211, an acid-type metabolite of 1,6-dimethyl-3-carbethoxy-4-oxo-6,7,8,9-tetrahydro-4H-pyrido-(1,2-alpha)-pyrimidin-1-ium-methyl-sulphate (MZ 144, Probon). Binding of MZ 144 to chemically modified albumins was studied under standard conditions. Binding studies with acetylated and methylated HSA indicated that the presence of free carboxyl groups is a requirement for binding, while the amino groups play a less important role. More selective chemical modifications of HSA were realised with acetylsalicylic acid, diethylpyrocarbonate (DEP) and o-nitrophenylsulfenyl-chloride (NPS-C1). Modifications with DEP and acetylsalicylic acid result in an increase of affinity in the binding area, probably due to blocking of a particular amino group in the vicinity of the binding site. Histidine residues are believed to be of minor importance in the binding process. When the indole ring is modified with NPS-C1, affinity at the strong binding site is significantly reduced, indicating that the Trp residue must be involved in the binding of MZ 144.
