Cytochrome syntheses

Cytochrome synthesis was examined in the fat body of adult male B. discoidalis by measuring the synthesis of cytochrome hemes. Heme is synthesized from the condensation of succinate and glycine by aminolevulinic acid synthase to produce aminolevulinic acid (ALA), a specific heme precursor. A developmental pattern exists for the incorporation of [i CjALA into cytohemes of fat body mitochondria with a peak of synthesis between days 4 and 6 of adult age (60). CC ablation eliminates this peak of synthesis for cytohemes a and b CC extract injections return the synthesis of cytohemes a+b to normal levels in CC-ablated cockroaches but have no effects on the synthesis of the c-type hemes for cytochromes c and Cj, The synthesis of cytohemes a+b in response to CC extracts requires a latent period of 24-48 hr to obtain a maximum response and is dose-dependent over a range of 0.01 to 0.08 CC pair (61). The active factor in CC extracts is sensitive to chymotrypsin but not to trypsin. This "cytochromogenic hormone (CGH) is secreted on days 2-3 of adult age in males (62). Since maximal synthesis of cytohemes a+b occurs on day 4, CGH secretion on days 2-3 agrees with the earlier observation that CGH requires about 48 hr to produce its response (61). 

Yeast is of exceptional value in the study of cytochrome regulation. It is a facultative anaerobe, and the usual cytochromes disappear under anaerobic conditions. On aeration, cytochrome induction can be demonstrated in the living cell, and the role of oxygen and carbon source, as well as the effect of inhibitors, can be evaluated by a variety of means [19-25], Yeast affords opportunities for genetic analysis and lends itself to studies of the inheritance of nuclear factors governing cytochrome synthesis. Moreover, this organism readily develops a mitochondrial abnormality on treatment with acriflavine, the petite mutation, which permits examination of the role of cytoplasmic inheritance [26]. 

Evidence is also available that higher organisms have a mitochondrial protein-synthesizing system inner membrane of rat liver mitochondria shows chloramphenicol-sensitive amino acid incorporation [66,112], and, in the intact animal, cytochrome synthesis is indicated to be inhibited by chloramphenicol [113]. HeLa cells appear to respond similarly [114]. However, ribosomal RNA of the mitochondria of HeLa cells, unlike that of fungal mitochondria, does not seem to be coded for by mitochondrial DNA [115], in harmony with indications that the informational content of DNA from animal mitochondria [116,117] is less than that of DNA from fungal mitochondria [118-120]. 

The formation of cytochromes is strongly affected by the availability of iron and copper. In the iron-deficient rat, for instance, cytochrome c can be decreased to less than half the normal value in skeletal muscle [125,126] or intestinal mucosa [126] cytochrome oxidase activity, however, is not markedly altered [127,128]. On administration of iron, the usual concentrations of cytochrome c are restored within 2 days in intestinal cells, and within 40 days in skeletal muscle. These restoration rates parallel the rates of cell renewal in these tissues [129]. In the copper-deficient rat, various tissues show greatly decreased levels of cytochrome oxidase [127,130-132], but (with one exception) normal levels of cytochrome c [131,132]. When copper is supplied, the rate of restoration of cytochrome oxidase level can exceed the rate of cell renewal in such tissues as liver and skeletal muscle [131]. The effect of iron or copper deprivation on cytochrome synthesis had originally been demonstrated in yeast [133]. 

In liquid medium, P. shermanii and P. freudenreichii grew more slowly under aeration than P. pentosaceum or P. rubrum (de Vries et al., 1972), and the extent of inhibition of the cytochrome synthesis was higher in strains... 

Table 3.5. Influence of aeration on growth and cytochrome synthesis by propionic acid bacteria...

Organism Growth conditions Growth rate (AE66(/h) Differential rate of cytochrome synthesis (pmol/g dry weight)... 

The lack of synthesis of the holocytochrome oxidase under anaerobic conditions may be due either to the repression of synthesis of some components in the absence of oxygen or to the inability to form some key intermediate directly requiring oxygen. As the technology is now available to examine the synthesis of subunits of this multicomponent complex, further investigations in this area will be of considerable interest in terms of the mechanism of the anaerobic repression of cytochrome synthesis. 

This is the reverse Pasteur or Crabtree effect and is also known as glucose inhibition or cataboHte repression. In the presence of higher sugar concentrations, synthesis of respiratory enzymes such as cytochromes is inhibited. 

The most conspicuous use of iron in biological systems is in our blood, where the erythrocytes are filled with the oxygen-binding protein hemoglobin. The red color of blood is due to the iron atom bound to the heme group in hemoglobin. Similar heme-bound iron atoms are present in a number of proteins involved in electron-transfer reactions, notably cytochromes. A chemically more sophisticated use of iron is found in an enzyme, ribo nucleotide reductase, that catalyzes the conversion of ribonucleotides to deoxyribonucleotides, an important step in the synthesis of the building blocks of DNA. 

FIGURE 22.17 The R. viridis reaction center is coupled to the cytochrome h/Cl complex through the quinone pool (Q). Quinone molecules are photore-duced at the reaction center Qb site (2 e [2 hv] per Q reduced) and then diffuse to the cytochrome h/ci complex, where they are reoxidized. Note that e flow from cytochrome h/ci back to the reaction center occurs via the periplasmic protein cytochrome co- Note also that 3 to 4 are translocated into the periplasmic space for each Q molecule oxidized at cytochrome h/ci. The resultant proton-motive force drives ATP synthesis by the bacterial FiFo ATP synthase. (Adapted from Deisenhofer, and Michel, H., 1989. The photosynthetic reaction center from the purple bac-terinm Rhod.opseud.omoaas viridis. Science 245 1463.)... 

In the endoplasmic reticulum of eukaryotic cells, the oxidation of the terminal carbon of a normal fatty acid—a process termed ch-oxidation—can lead to the synthesis of small amounts of dicarboxylic acids (Figure 24.27). Cytochrome P-450, a monooxygenase enzyme that requires NADPH as a coenzyme and uses O, as a substrate, places a hydroxyl group at the terminal carbon. Subsequent oxidation to a carboxyl group produces a dicarboxylic acid. Either end can form an ester linkage to CoA and be subjected to /3-oxidation, producing a... 

Chemical synthesis of cytochrome P-450-dependent metabolites (epoxyeico-satriene acids and other metabolites possessing heterocyclic fragments) 98MI9. 

Zhong W, Uss AS, Ferrari E, Lau JY, Hong Z (2000) De novo initiation of RNA synthesis by hepatitis C virus nonstructural protein 5B polymerase. J Virol 74 2017-2022 Zhou S, Yung Chan S, Cher Goh B, Chan E, Duan W, Huang M, McLeod HL (2005) Mechanism-based inhibition of cytochrome P450 3A4 by therapeutic drugs. Clin Pharma-cokinet 44 279-304... 

Mechanistic studies have shown that TBT and certain other forms of trialkyltin have two distinct modes of toxic action in vertebrates. On the one hand they act as inhibitors of oxidative phosphorylation in mitochondria (Aldridge and Street 1964). Inhibition is associated with repression of ATP synthesis, disturbance of ion transport across the mitochondrial membrane, and swelling of the membrane. Oxidative phosphorylation is a vital process in animals and plants, and so trialkyltin compounds act as wide-ranging biocides. Another mode of action involves the inhibition of forms of cytochrome P450, which was referred to earlier in connection with metabolism. This has been demonstrated in mammals, aquatic invertebrates and fish (Morcillo et al. 2004, Oberdorster 2002). TBTO has been shown to inhibit P450 activity in cells from various tissues of mammals, including liver, kidney, and small intestine mucosa, both in vivo and in vitro (Rosenberg and Drummond 1983, Environmental Health Criteria 116). 

Trager, W.F. (1988). Isotope effects as mechanistic probes of cytochrome P450-catalysed reactions. In Synthesis and Application of Isotopically Labelled Compounds Proceedings of the Third International Symposium T.A. Baillie and J.R. Jones (Eds.) Amsterdam Elsevier 333-340. 

Many drugs when administered to humans can result in a marked increase in ALASl. Most of these drugs are metabolized by a system in the liver that utilizes a specific hemoprotein, cytochrome P450 (see Chapter 53). During their metabolism, the utilization of heme by cytochrome P450 is greatly increased, which in turn diminishes the intracellular heme concentration. This latter event effects a derepression of ALASl with a corresponding increased rate of heme synthesis to meet the needs of the cells. 

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