True inhibition

Table 7.1 Measured and true inhibition parameters for a hypothetical series of compounds when measured in an assay for which the enzyme concentration is 50 nM...

Inhibition of the Oxidase Activity. The oxidase activity of ceruloplasmin has been reported to be inhibited by a wide variety of different substances, and it would be well to briefly point out several ways in which an Inhibition of the activity may be manifested (a) A true inhibition of the activity resulting from a combination of enzyme or an enzyme-substrate complex with the effector, (b) Inhibition resulting from the sequestering of free iron, as has already been discussed, (c) Alteration of the chromophoric properties of the substrate or product by the effector, for example, the reduction of the colored radical cation of DPD. (d) Apparent inhibition will also result when the inhibitor causes aggregation or precipitation of the protein. Inhibition attributable to each of the above has been reported, [cf. Curzon and Speyer (175) for a lucid discussion of these points.] We will be concerned here primarily with three types of inhibitors, all of which act by a direct combination with some form of the enzyme halide and inorganic anions, carboxylic acids having a double bond a- to the carboxyl function, and certain divalent cations. 

True inhibition is usually applicable only to intermolecular reactions rather than decompositions. The polymerization of vinyl compounds is an example when a runaway is detected, an inhibitor such as tertiary butyl catechol can be added to stop the reaction by removing the free radicals which propagate it. This method can be applied to other types of reaction, but a laboratory study is always necessary to select the inhibitor and conditions such as inhibitor concentration, mixture temperature and speed of addition. The reliability of the inhibition system should be quantified and demonstrated to be acceptable if inhibition is to be used as the ultimate basis of safety. 

Excess substrate inhibited the oxidation of the p-hydroxyphenyl-pyruvate as did also benzoquinone acetic acid. This could be prevented by adding large amounts of ascorbic acid or small amounts of reduced 2,6-dichlorophenolindophenol. The latter was seven hundred times as effective as ascorbic acid with the purified enzyme 214), Zannoni and LaDu speculate that a product formed from p-hydroxyphenylpyruvate is the true inhibiting agent of the reaction. 

In order to see the true inhibition in the potential region where the current decreases with potential, a correction has to be made for the gradient of the concentration of H2 in the diffusion layer. The partial... 

Metals that produce protective oxide layers (such as stainless steels) are especially susceptible to crevice attack. The reduced oxygen concentration in the crevice inhibits repair of the protective oxide film. This is especially true if acidic anions are present, which further retards oxide repair. Stainless steels containing molybdenum are usually less susceptible to attack. 

Ph. C CH. C(CH,. N(Br)Mc3) C(0. CO. NMca). CH CH, an analogue of prostigmine, shows marked inhibition for i/i-choline-esterase, but is only slightly active towards true choline-esterase. 

These faets, however, cannot be eorreedy studied without taking into account a considerable amount of eollateral matter. For example, whilst the aetual pereentage of esters in the essential oils is inereased by the use of sodium ehloride, this salt has an inhibiting aetion on the vegetation generally, so that the aetual weight of methyl esters per acre is less than when no sodium ehloride is used, whilst the reverse is true when sodium nitrate is used. 

The proof of protection is more difficult to establish in this case for two reasons. First, the object is to restore passivity to the rebar and not to render it virtually immune to corrosion. Second, it is difficult to measure the true electrode potential of rebars under these conditions. This is because the cathodic-protection current flowing through the concrete produces a voltage error in the measurements made (see below). For this reason it has been found convenient to use a potential decay technique to assess protection rather than a direct potential measurement. Thus a 100 mV decay of polarisation in 4 h once current has been interrupted has been adopted as the criterion for adequate protection. It will be seen that this proposal does not differ substantially from the decay criterion included in Table 10.3 and recommended by NACE for assessing the full protection of steel in other environments. Of course, in this case the cathodic polarisation is intended to inhibit pit growth and restore passivity, not to establish effective immunity. 

In many cases, problems cannot be overcome by biological means. This is especially true for those related to inhibition by substrate or product. There may, however, be technical solutions to these problems. Nowadays, complicated feed strategies with different substrates can be achieved through the use of flow injection analysis, on-line sensors, mass flow meters and sophisticated computer control. Such control coupled to a fed-batch mode of operation (Figure 2.5) can often eleviate problems caused by substrate inhibition. For some processes, continuous product removal can avoid the problems associated with product inhibition the various options include ... 

Helf White (Ref 2) interpret the above behavior of the nitrocompds in inhibiting the scintillation process as one of simple light absorption rather than as a true chemical quenching (ae-excitation process). To substantiate this, the UV and near-visible spectrum of each of the light compds in toluene—PPO soln was measured using the 50% extinction concn for each nitrocompd (as determined from Fig 1). 

A particle size effect has been detected by Chou and Olson [486] in the isothermal decomposition of isothiocyanatopentammine cobalt(III) perchlorate. Below a = 0.09, the larger crystals decompose relatively more rapidly than the smaller, whereas for a > 0.09, the reverse is true. This behaviour was attributed to enhanced nucleation in the larger particles due to strain, but this favourable factor was later offset by the inhibiting influence of the product ammonia which accumulated in the larger crystals. 

One form of biological poisoning mirrors the effect of lead on a catalytic converter. The activity of an enzyme is destroyed if an alien substrate attaches too strongly to the enzyme s active site, because then the site is blocked and made unavailable to the true substrate (Fig. 13.42). As a result, the chain of biochemical reactions in the cell stops, and the cell dies. The action of nerve gases is believed to stem from their ability to block the enzyme-controlled reactions that allow impulses to travel through nerves. Arsenic, that favorite of fictional poisoners, acts in a similar way. After ingestion as As(V) in the form of arsenate ions (As043 ), it is reduced to As(III), which binds to enzymes and inhibits their action. 

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