Inhibition effects Subject

Another situation is observed when salts or transition metal complexes are added to an alcohol (primary or secondary) or alkylamine subjected to oxidation in this case, a prolonged retardation of the initiated oxidation occurs, owing to repeated chain termination. This was discovered for the first time in the study of cyclohexanol oxidation in the presence of copper salt [49]. Copper and manganese ions also exert an inhibiting effect on the initiated oxidation of 1,2-cyclohexadiene [12], aliphatic amines [19], and 1,2-disubstituted ethenes [13]. This is accounted for, first, by the dual redox nature of the peroxyl radicals H02, >C(0H)02 and >C(NHR)02 , and, second, for the ability of ions and complexes of transition metals to accept and release an electron when they are in an higher- and lower-valence state. 

At a first glance, this controller is sufficient for maintaining the product purity. However, simulation results indicate that, in order to maintain x-q at the desired level when the system is subjected to a small (5%) increase in the mole fraction yi o, the recycle flow rate R would need to rise to 501.3mol/ min (a fivefold increase from the nominal value). Thus, due to its inhibitive effect on the reaction rate, the accumulation of the impurity I is highly detrimental to the operation of the process. Consequently, the control of the impurity levels in the reactor is of critical importance and directly linked to the main objective of product-purity control. 

The kinetics and mechanism of methane combustion have been the subject of many investigations, e.g.. Refs. 43-47, because of the importance of natural gas as a potential fuel for catalytic combustors. Under conditions expected in catalytic combustors, i.e., excess oxygen, a first order in methane is generally observed [48], whercas a variety of orders has been observed for other hydrocarbons [13]. The actual mechanism appears to be quite complex and depends on the fuel used. For instance, inhibiting effects are observed for the products carbon dioxide and water in methane combustion over supported palladium catalysts [49,50]. The inhibition of methane adsorption and the formation of a surface palladium hydroxide were proposed to explain the observation. 

Two typical phase I dmg interaction clinical trial designs are shown in Figure 20.4. Pharmacokinetic profiles are found within each subject for each substrate with and without concomitant exposure to the study dmg. These designs are applicable to both enzyme induction and inhibition effects (Figure 20.4, upper half). The same study schematic can be used to study the effects of inhibitors or inducers on the study dmg as a substrate for CYP450 systems (as illustrated in the lower half of Figure 20.4). 

Because many of the medications used to treat anxiety are addictive, it is essential to screen the client for potential substance abuse. It should come as no surprise to social workers that many people who suffer from anxiety often seek relief by using drugs or alcohol. Similar to the medications used to treat anxiety, alcohol and some of the recreational street drugs can stimulate the release of certain neurochemicals in the brain that inhibit anxiety. Subjectively, the result of this biochemical process is social ease, experienced as pleasure, that seduces the users of chemical intoxicants to continue despite the side effects slurred speech, slowed thoughts, memory failure, poor motor control, and the possibility of addiction (Marshall. 1994, p. 152). If a history of substance abuse or a tendency to abuse prescription medication is noted, it is important to flag this potential problem. 

To assess selectivity in the inducing and inhibiting effects of environmental factors (for example, dmg treatment) on the activity of the different P-450 enzymes. In one of such smdies, nifedipine, sparteine, mephenytoin and antipyrine were administered simultaneously to 15 healthy subjects, including four poor metabohzers of sparteine and four poor metabolizers of mephenytoin [6], They received the cocktail on three different occasions without pretreatment, after pentobarbital pretreatment and together with cimetidine. Concentrations of nifedipine, its pyridine metabolite, and of sparteine and its dehydro metabolite were measured in the plasma nifedipine metabohtes, sparteine, dehydrosparteine, 4-hydroxymephenytoin, antipyrine and its three major metabohtes were all measured in urine. The kinetic parameters obtained clearly indicated that nifedipine metabolism is very sensitive to pentobarbital induction (Fig. 4), whereas antipyrine metabolism is sensitive only to a moderate degree and sparteine metabolism is unaffected. Cimetidine inhibited the metabohc clearance of all three compounds to a similar extent and also inhibited renal clearance of sparteine, most likely at the level of tubular secretion. Urinary excretion of 4-... 

A study in 10 healthy subjects found that ticlopidine 250 mg twice daily for 3 weeks decreased the clearance of phenazone (a marker of enzyme inhibition or induction). The AUC increased by 14% and the half-life increased by 27%, suggesting that ticlopidine has some mild enzyme-inhibiting effects. This is consistent with the way ticlopidine appears to inhibit the metabolism of theophylline , (p.ll77), but so far no other drugs seem to be affected to a clinically important extent. 

An excellent example of allosteric regulation—the control of an allosteric enzyme—is the five-step synthesis of the amino acid isoleucine (see I Figure 10.13). Threonine deaminase, the enzyme that catalyzes the first step in the conversion of threonine to isoleucine, is subject to inhibition by the final product, isoleucine. The structures of isoleucine and threonine are quite different, so isoleucine is not a competitive inhibitor. Also, the site to which isoleucine binds to the enzyme is different from the enzyme active site that binds to threonine. This second site, called the allosteric site, specifically recognizes isoleucine, whose presence there induces a change in the conformation of the enzyme such that threonine binds poorly to the active site. Thus, isoleucine exerts an inhibiting effect on the enzyme activity. As a result, the reaction slows as the concentration of isoleucine increases, and no excess isoleucine is produced. When the concentration of isoleucine falls to a low enough level, the enzyme becomes more active, and more isoleucine is synthesized. This type of allosteric regulation in which the enzyme that catalyzes the first step of a series of reactions is inhibited by the final product is called feedback inhibition. 

The inhibiting effect of sodium silicate has been known since 1922 [1] and has been the subject of many studies [2, 3]. 

The degree of tack-free cure is to some extent subjective and many of the later-generation UV adhesives will be less susceptible to the oxygen inhibition effect. UV cyanoacrylates and UV epoxies, for example, will give excellent tack-free finishes even under relatively low-intensity UVA lamps. 

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