title: 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.
mesh_terms: D003306:Coproporphyrins / Q000378:metabolism; D005982:Glutathione Transferase / Q000378:metabolism*; D006415:Hematoporphyrins / Q000378:metabolism; D046911:Macromolecular Substances; D011166:Porphyrins / Q000378:metabolism*; D011485:Protein Binding; D011524:Protoporphyrins / Q000378:metabolism; D013057:Spectrum Analysis; D013448:Sulfobromophthalein / Q000494:pharmacology; D014578:Uroporphyrins / Q000378:metabolism
chemical_list: D003306:Coproporphyrins; D006415:Hematoporphyrins; D046911:Macromolecular Substances; D011166:Porphyrins; D011524:Protoporphyrins; D014578:Uroporphyrins; D013448:Sulfobromophthalein; D005982:Glutathione Transferase