Heme-containing systems, activation

Lead has been shown to decrease circulating levels of the active form of vitamin D (1,25-dihydroxy-vitamin D) in children. The conversion of vitamin D to this active hormonal form takes place via hydroxylation to 25-hydroxyvitamin D in the liver, followed by 1-hydroxylation in the mitochondria of the renal tubule by a complex cytochrome P-450 (heme-containing) system (Mahaffey et al. 1982 Rosen and Chesney 1983). Comparisons of the serum 1,25-dihydroxyvitamin D levels in children with blood lead levels of 33 g/dL with those in children with severe renal insufficiency (Rosen et al. 1980) and in children with an inborn error of vitamin D metabolism in which the 1-hydroxylase system or component... 

In the case of heme-containing systems, it is believed that the activation barrier for 0-0 bond cleavage can be lowei by the complexation of the resulting oxygen atom by the iron porphyrin center, i.e. ... 

Most other peroxidases are Fe-heme-containing systems, which function as two-electron redox catalysts (Scheme 8). Dihydrogen peroxide oxidizes the Fe-heme moiety by two electrons, forming Compound 1 (a heme + FeIV=0 species) [97], Compound 1 oxidizes the halide ion, forming the active halogenating species. This mechanism cannot be operative in V-BrPO because the vanadium is already in its highest accessible oxidation state. Moreover, native V-BrPO does not oxidize bromide without an acceptable peroxide source. However, it should... 

A comparison of porphyrin and pincer activity rationalized through reactivity index Porphyrin and pincer complexes are both important categories of compounds in biological and catalytic systems. Structure, spectroscopy, and reactivity properties of porphyrin pincers are systematically studied for selection of divalent metal ions. It is reported that the porphyrin pincers are structurally and spectroscopically different from their precursors and are more reactive in electrophilic and nucleophilic reactions. These results are implicative in chemical modification of hemoproteins and understanding the chemical reactivity in heme-containing and other biologically important complexes and cofactors [45]. 

The cytochrome P450 system is the principal enzyme system for the metabolism of lipophilic xenobiotics. It is a heme-containing, membrane-bound, multi-enzyme system which is present in many tissues in vivo but is present at the highest level in liver. A coenzyme, cytochrome P450 NADPH oxidoreductase (OR), is essential for P450 catalytic function and cytochrome bs may stimulate catalytic activities of some enzymes. In human liver, it is estimated that there are 15-20 different xenobiotic-metabolizing cytochrome P450 forms. A standard nomenclature, based on relatedness of the amino acid sequences, has been developed (Nelson et al., 1993). The most recent... 

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