Apohemoglobin (apoHb) was reconstituted with various iron porphyrins (FePs) such as hemin, TCPP-Fe, TPPS4-Fe and TTMAPP-Fe. The binding constants and binding ratios of apoHb and FePs were determined by fluorescence quenching method. TCPP-Fe, TPPS4-Fe and hemin (the nature prosthetic group in Hb) are metalloporphyrins (MPs) with negatively charged porphyrin ring. All of them bind to apoHb with a binding ratio of 3. However, the positively charged TTMAPP-Fe binds to apoHb with a binding ratio of 2. The binding constants of hemin, TCPP-Fe, TPPS4-Fe and TTMAPP-Fe were 2.72 x 10(6), 1.17 x 10(6), 5.17 x 10(5) and 1.6 x 10(5), respectively. The Soret band of the reconstituted hemoglobin showed evident changes in the presence of H2O2. The peroxidase activities of these complexes were determined. The results showed that though apoHb could be reconstituted with all FePs, it showed different functions. Binding of apoHb with hemin can enhance the heterolytic cleavage of O-O bond of H2O2, but inhibiting the cleavage when TCPP-Fe and TPPS4-Fe were used instead. The binding of apoHb with TTAMPP-Fe showed negligible influence on the cleavage of O-O bond in H2O2. These different results were discussed. (C) 2004 Elsevier B.V. All rights reserved.