THERMODYNAMICS OF THE BINDING OF HEMATOPORPHYRIN ESTER, A HEMATOPORPHYRIN DERIVATIVE‐LIKE PHOTOSENSITIZER, AND ITS COMPONENTS TO HUMAN SERUM ALBUMIN, HUMAN HIGH‐DENSITY LIPOPROTEIN AND HUMAN LOW‐DENSITY LIPOPROTEIN

The phenomena of the high affinity of porphyrins to the human serum proteins, albumin, high‐density lipoproteins (HDL) and low‐density lipoproteins (LDL) is well established. Yet, evaluation of the activities of these proteins as endogenous porphyrin carriers, especially with respect to receptor‐mediated porphyrin uptake into tumor cells, the merits of which are still in dispute, requires more quantitative protein‐porphyrin binding data. As a continuation of previous studies on this issue, the binding of several porphyrin systems to each of the three proteins, employing previously developed spectral methodologies, was studied. The specific systems reported here are hematoporphyrin ester (HPE), which is a novel hematoporphyrin derivative (HPD)‐like system, two porphyrin trimers (denoted O1 and O2) and a porphyrin dimer (denoted O3) isolated from HPE. Human serum albumin (HSA) was found to have a single high‐affinity site for the monomeric components of HPE, with an equilibrium binding constant of 3.6 × 10⁶. The equilibrium parameters determined for the binding of the three HPE‐isolated oligomers to each of the serum proteins are: (1) Binding constants (K_b') of 2.3 × 10⁶, 6.9 × 10⁴ and 1.5 × 10⁴ and number of sites per protein molecule (n) of 3, 1 and 5, for the binding of 01, 02 and 03, respectively, to HSA. (2) K_b’values of 15.5 × 10³, 15.3 × 10³ and 6.6 × 10³ and n values of 1, 2 and 2, for the binding of O1, O2 and O3, respectively, to HDL. (3) K_b’values of 3.3 × 10³, 2.28 × 10⁴ and 8.0 × 10³ and n values of 50, 20 and 16 for the binding of O1, O2 and O3, respectively, to LDL. These data are direct and clear support not only for the high affinity of porphyrins to serum proteins but specifically of stable oligomers that have been assigned critical roles in the photodynamic treatment of tumors. Of the three proteins, LDL is clearly the best camer, providing the highest drug payload with a moderate affinity (enough to bind and not too much to prevent release). These data are suggested to be promising for the postulated role of LDL in porphyrin uptake into tumor cells and to be useful in the future as benchmarks for novel porphyrin systems.