Difference in the transport of metal and free-base porphyrins Steady-state and time-resolved fluorescence studies

The binding of Mg-mesoporphyrin and mesoporphyrin to the primary binding site of human serum albumin (HSA) has been studied in the absence and in the presence of 1,2-dimyristoyl-sn-glycero-3- phosphatidylcholine/1,2-dimyristoyl-sn-glycero-3-phosphatidylglycerol (DMPC/DMPG) liposomes. The equilibrium constants of Mg-mesoporhyrin IX (MgMP) and mesoporphyrin IX (MP) for association to HSA were 1.7 . 10(7) (M-1) and 2.5 . 10(7) (M-1), respectively. The association constants for binding to the liposomes were: 1.5 . 10(4) (M-1) for MgMP, and 3.2 . 10(4) (M-1) for MP. The smaller value for the association constants of the MgMP relative to MP in both processes are interpreted as the effect of an out of plane position of Mg2+ and possible ligand co-ordination. HSA added to the liposomes with incorporated porphyrins results in the redistribution of bound molecules. For the MgMP this distribution can be interpreted by the competition of two independent binding processes, while the binding kinetics of MP significantly deviates from this model. The difference is explained by supposing a specific interaction between HSA and the liposomes in case of the free-base MP. (C) 1998 Elsevier Science B.V. All rights reserved.