Constitutive enzyme system

Monooxygenases (MOs) Enzyme systems of the endoplasmic reticulum of many cell types, which can catalyze the oxidation of a great diversity of lipophilic xenobiotics, are particularly well developed in hepatocytes. Forms of cytochrome P450 constitute the catalytic centers of monooxygenases. 

The NADPH oxidase is in fact a multicomponent enzyme system that constitutes an electron transport chain from NADPH to O2. The components of this oxidase complex are now almost completely defined, and experiments performed primarily with CGD neutrophils have helped to identify these major constituents. 

The enzymatic system participating in degradation of TNT or other nitroaromatics in plants has not yet been sufficiently characterised. Based on recent results, TNT is not metabolised by highly specific nitroreductases, which would be purposefully synthesized by plants, but by constitutive enzymes with nitroreductase activity (Nepovim 2005b). This assumption is in agreement with results presented by Ekman (2005) showed an increase of the expression of a couple of enzymes in root mRNA due to the induction caused by explosives. 

The present reaction was proven to occur even when the microorganism had been grown on peptone as the sole carbon source. These results lead to the conclusion that this enzyme system is produced constitutively. In the case of man-delate-pathway enzymes in Pseudomonas putida, (S)-mandelate dehydrogenase was shown to be produced in the presence of an inducer (mandelic acid or benzoylformic acid) [5]. Thus, the expression of the present oxidizing enzyme of A. bronchisepticus is different from that of R putida. 

The lytic enzyme systems, active against yeast cell walls, usually contain l,3-/ -glucanases, proteases, mannanases, chitinases, and 1,6-) -glucanases. The proportion of those enzyme activities, their action pattern, synergism, and dependence on inhibitors constitute the activity profile... 

The major polymers that make up the wall are polysaccharides and lignin. These occur together with more minor but very important constituents such as protein and lipid. Water constitutes a major and very important material of young, primary walls (2). The lignin is transported in the form of its building units (these may be present as glucosides) and is polymerized within the wall. Those polysaccharides which make up the matrix of the wall (hemicelluloses and pectin material) are polymerized in the endomembrane system and are secreted in a preformed condition to the outside of the cell. Further modifications of the polysaccharides (such as acetylation) may occur within the wall after deposition. Cellulose is polymerized at the cell surface by a complex enzyme system transported to the plasma membrane (3). 

The studies on the methylation of dihydroxybenzaldehyde and the earlier studies on the decarboxylation of oxaloacetic acid illustrate a hypothesis about metal-catalyzed enzymes that is not proved but has been substantiated in a number of instances in which it has been tried. The hypothesis is that, if a metal constitutes the active site of an enzyme, it should be possible to carry out the reaction with metal ions alone in the absence of the enzyme. The rates of non-enzymatic reactions may be much lower, and the metal ions may be more active metal ions than those that activate the enzyme, for the reasons already discussed. This hypothesis is the basis for much of the work on metal catalytic reactions that are models for enzyme systems. 

The stimulation of the synthesis of the cellulase system of T. reesei QM 9414 by sophorose was established as shown by the results of experiments summarized in Table IV. Other than sophorose, of the glycosides and oligosaccharides tested, only lactose caused even a limited production of the enzymes of the cellulase system. Lactose is not as closely related structurally to sophorose as is, for example, the disaccharide laminaribiose it is more closely related structurally to cellobiose, which, despite being the major product of cellulose breakdown, does not promote enzyme production under the conditions of this experiment. It was noted that both intra- and extracellular constitutive enzyme levels produced by cells growing on glucose (or by resting cells without inducer, Table IV) are less than 0.5% of the fully induced levels and thus are negligible. 

More than 1 kg of purified cis-(lS,2R)-indanediol was produced and incorporated into indinavir sulfate, the active ingredient of Crixivan . The undesired necessity to induce the dioxygenase with toluene was by-passed through mutation rounds to screen for a constitutive producer of the TDO enzyme system solely... 

Evidence is similar for humans but limited, and includes male sterility, spontaneous abortions in human females, premature human fetuses, severe neurologic and CNS effects, blood dyscrasias, hepatotoxicity, accumulation of organohalogen pesticides in human lipid tissue—and, perhaps even more important, their presence in human breast milk, whence they can continue to exert influences on growth, development and hormonal, CNS and enzyme systems. Aldrin, dieldrin, chlordane, chlordecone (Kepone), heptachlor epoxide, hexachlorobenzene (HCB) and Mirex are all excreted via breast milk in the human female. This is also true for the related PCBs and PBBs that resist biodecomposition and maintain persistent residence in mammalian tissues. For them, excretion via breast milk may constitute the main—if not sole—elimination route. 

The enzyme systems needed for bacterial degradation of dibenzothiophene usually require an inducer which may be the substrate itself, naphthalene, anthracene or salicylate (59. 60). However, Pseudomonas sp. HL7b co-metabolizes dibenzothiophene constitutively (33). That is, a glucose-grown suspension of HL7b cells immediately started to oxidize dibenzothiophene when it was added to the medium and yielded HFBT among other metabolites. 

Superoxide dismutases (SOD EC 1.15.1.1) constitute a group of metalloisoenzymes that neutralise the very reactive superoxide radical, generated in the cell as a by-product of the reduction of molecular oxygen (Salin, 1988). SOD is a highly efficient enzyme system involved in the cellular antioxidant system (cf. section on defence mechanisms). 

The Ca2+ released from the endoplasmic reticulum by IP3 may perform a number of functions. The metabolically most important action of Ca2+, however, is to combine with a very ubiquitous protein called calmodulin. This protein has a molecular weight of about 17,500 and is in many ways similar to troponin. It has four Ca2+ binding sites and constitutes one of several subunits of several enzyme systems. Thus, as cellular [Ca2+] rises, the calmodulin subunit binds Ca2+. The result is that it changes its conformation to that of an a helix and thereby affects the catalytic activity of other constituent subunits. For instance, calmodulin is a component of myosin kinase, which phosphorylates one of the subunits of myosin. The structure of calmodulin is shown in Figure 16.22. 

The isolation48 of epoxyfarnesal (8), along with dihydroxyfarnesal (9) and dihyd-roxyfarnesol (10), when trans,trans-epoxyfarnesol (11) was converted into cis,trans-epoxy-farnesol (12) by an enzyme system from H. sativum supports the view that aldehyde intermediates are involved in the isomerization sequence, but this does not constitute a demonstration of this route since the aldehyde may be involved in a side-reaction or a subsequent-reaction. 

In the liver, isoprenaline-stimulated adenylate cyclase has been found to be located almost exclusively on the surface of the parenchymal cells, with little or no deposit on the surface of the reticulo-endothelial cells. In contrast, the predominant effect of glucagon resulted in the deposition of reaction product on the reticulo-endothelial cell surface, although deposits were also present on the parenchymal cells. In the presence of F , there were substantial deposits on the surface of both types of cells. These results demonstrate that distinct enzyme systems are present in parenchymal and reticulo-endothelial cells. No theories concerning the function of adenylate cyclase in the liver have considered its role in the endothelial cells which act mainly on phagocytes. These cells contribute 35% of the cells in the liver and can be expected to contribute substantially to biochemical measurements of adenylate cyclase activity and cyclic AMP concentrations when liver slices, homogenates, or cell fractions constitute the enzyme source. 

---