The binding of porphyrins by ligandin

Abstract

Spectrophotometric and equilibrium-dialysis measurements show that ligandin (glutathione S-transferase B, EC 2.5.1.18) binds monomeric porphyrins at a single site with association constants in the range 10(4)-10(6) litre/mol at pH 7.0. Binding affinities are paralleled by the tendencies of the porphyrins to aggregate, increasing in the order: uroporphyrins I and III less than coproporphyrins I and III approximately haematoporphyrin less than protoporphyrin IX. From this it is deduced that the hydrophobic effect is the predominant driving-force for binding. The porphyrins can be displaced from their binding site on ligandin by bromosulphophthalein and oestrone sulphate. In enzyme inhibition studies, 50% inhibition was brought about by 8 micron-haematoporphyrin and by 1 micron-protoporphyrin IX. In the analysis of the haemotoporphyrin-ligandin system the self-association of haematoporphyrin was studied in detail. It was found to be limited to dimerization in the concentration range 0-200 micron at pH 7.0, 25 degrees C and a dimerization constant of 1.9 x 10(5) litre/mol was determined. Coproporphrin III has a dimerization constant of 5.2 x 10(5) litre/mol under the same conditions.

The role of glutathione-S-transferase in anti-cancer drug resistance

“…The first GST described was originally identified as 'ligandin' due to its ability to interact covalently and noncovalently with various compounds that are not substrates for enzymatic activity, including steroids, thyroid hormones, bile acid, bilirubin, and heme (Litwack et al, 1971;Tipping et al, 1978;Danielson and Mannervik, 1985). While the ligand-binding function remains unclear, sequestering molecules may serve a regulatory role, preventing cytotoxic ligands from interacting with their targets.…”

The Influence of Aggregation of Porphyrins on the Efficiency of Photogeneration of Hydrogen Peroxide in Aqueous Solution

“…This was done by referring to typical standard spectra assigned previously to a monomeric form, another type of monomeric form protonated at the inner nitrogen atoms of monomeric porphyrin (abbreviated as a N-protonated monomer), and a dimeric form. [8][9][10][11][12] Relative concentrations of individual species were calculated with a computer program developed in our laboratory by fitting the sum of the fractions of the standard spectra of the pure forms to the spectrum observed.…”

“…9,10,12) First, we used CP, because this porphyrin was in a monomer-dimer equilibrium at around a neutral pH, 8,10,11) and expected to show strong pH-dependence. To obtain information on the state of the aggregation of CP, we measured absorption spectra at various pHs.…”

“…8) As a continuation and extension of this work, we examined the effect of pH on the photogeneration of H 2 O 2 , because porphyrins are known to take different aggregated forms depending on their environment, including the pH of solution. [9][10][11][12] Coproporphyrin (CP) and uroporphyrin (UP) were chosen, because these porphyrins are known to photogenerate sufficient amounts of H 2 O 2 . [6][7][8] It was found that the efficiency in producing H 2 O 2 was strongly dependent on the aggregation of CP and UP mediated by changes in the pH of the solution, with a dimer suppressing, and a highly aggregated form enhancing, the production of H 2 O 2 .…”

Porphyrin-Induced Photogeneration of Hydrogen Peroxide Determined Using the Luminol Chemiluminescence Method in Aqueous Solution: A Structure-Activity Relationship Study Related to the Aggregation of Porphyrin

“…In this study, we used a luminol chemiluminescence method 9,10 and found that it could quantify H2O2 even in the presence of a high concentration of porphyrin. Using the present method, we demonstrated how the chemical properties of porphyrins, especially their aggregation in an aqueous solution, [11][12][13] influenced the photogeneration of H2O2.…”