Photodynamic crosslinking of proteins. I. Model studies using histidine- and lysine-containing...

One of the mechanisms by which cells might be damaged during the photodynamic therapy (PDT) of tumors is via the covalent crosslinking of proteins to proteins or to other molecules in the cell. It has been suggested that photodynamically generated singlet oxygen interacts with photo-oxidizable amino acid residues such as His, Cys, Trp and Tyr in one protein molecule to generate reactive species, which in turn interact non-photochemically with residues of these types or with free amino groups in another protein molecule to form a crosslink. In some cases, photochemically generated free radicals may be involved in crosslinking. This paper describes studies on the use of N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers containing epsilon-aminocaproic acid side chains terminating in His (P-Acap-His) or Lys (P-Acap-Lys) as models for the photodynamic crosslinking of proteins. The model copolymer P-Acap-His had a weight-averaged molecular weight of about 22,000 and contained four to five His residues per copolymer molecule. The model copolymer P-Acap-Lys had a weight average molecular weight of about 18,000 and contained four to five Lys residues per copolymer molecule. The extent of photocrosslinking, as sensitized by rose bengal, was estimated by measuring the increase in the viscosity of model copolymer solution after various periods of illumination. The extent of intermolecular crosslinking was estimated from the changes in molecular weight distribution of samples before and at the end of illumination as determined by size exclusion chromatography. Photodynamic crosslinking occurred between P-Acap-His molecules and between P-Acap-His and P-Acap-Lys molecules. The higher the concentration of macromolecules in the solution, the higher is the yield of intermolecular crosslinking. Oxygen was necessary for crosslinking, and azide inhibition studies indicated the involvement of singlet oxygen.