The binding of porphyrins by ligandin

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Abstract
Spectrophotometric and equilibrium-dialysis measurements show that [rat liver] ligandin (glutathione S-transferase B, EC 2.5.1.18) binds monomeric porphyrins at a single site with association constants in the range 104-106 l/mol at pH 7.0. Bindinf affinities are paralleled by the tendencies of the porphyrins to aggregate, increasing in the order: uroporphyrins I and III < coproporphyrins I and III .apprx. hematoporphyrin < 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 bromsulfophthalein and estrone sulfate. In enzyme inhibition studies, 50% inhibition was brought about by 8 .mu.M-hematoporphyrin and by 1 .mu.M-protoporphyrin IX. In the analysis of the hematoporphyrin-ligandin system the self-association of hematoporphyrin was studied in detail. It was limited to dimerization in the concentration range 0-200 .mu.M at pH 7.0, 25.degree. C, and a dimerization constant of 1.9 .times. 105 l/mol was determined. Coproporphyrin III has a dimerization constant of 5.2 .times. 105 l/mol under the same conditions.

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