Metabolic control Isoenzymes

The third and final control step is mediated by PK. This enzyme, like HK, exists as a number of isoenzymes in different tissues and, like the PFK reaction, is controlled by both the concentration of metabolites and covalent effects. Furthermore, PK also illustrates two other means of metabolic control, namely enzyme induction and feedforward, regulation. 

Hydrogenase isoenzymes are also common among the metabolically more versatile bacteria (see Chapter 2). For instance, H2 metabolism and isoenzyme composition in enteric bacteria, including Escherichia coli and Salmonella typhimurium, appear to be differentially regulated under the two modes of anaerobic life, fermentation and anaerobic respiration (Table 3.1). Furthermore, biosynthesis of the individual isoenzymes appears to be controlled at a global level by the quality of the carbon source. 

The metabolic control of methionine metabolism is complex and involves, for example, changes of enzyme levels in particular tissues, mechanisms linked to the kinetic properties of the various enzymes and their interaction with metabolic effectors [6, 7]. A particularly important metabolic effector is AdoMet. This inhibits the low Km isoenzymes of MAT, and MTHF reductase, inactivates betaine methyltransferase, but activates MAT III (the high-Km isoenzyme) and cystathionine /1-synthase. Therefore, high methionine intake and thus higher AdoMet levels favour trans-sulphuration, and when levels are low methionine is conserved. AdoHcy potently inhibits AdoMet-dependent methyltransferases and both Hey remethylating enzymes. Another important control mechanism is the export of Hey from cells into the extracellular space and plasma, which occurs as soon as intracellular levels increase [8]. 

As indicated in the previous discussion, the control of enzyme activity is understood in terms of kinetic parameters. Differences in Km and or Vmax can also arise when the same chemical (metabolic) reaction is catalysed by two structurally different enzymes. Such is the case with isoenzymes or isoforms of enzymes. 

Probably the most common and widespread control mechanisms in cells are allosteric inhibition and allosteric activation. These mechanisms are incorporated into metabolic pathways in many ways, the most frequent being feedback inhibition. This occurs when an end product of a metabolic sequence accumulates and turns off one or more enzymes needed for its own formation. It is often the first enzyme unique to the specific biosynthetic pathway for the product that is inhibited. When a cell makes two or more isoenzymes, only one of them may be inhibited by a particular product. For example, in Fig. 11-1 product P inhibits just one of the two isoenzymes that catalyzes conversion of A to B the other is controlled by an enzyme modification reaction. In bacteria such as E. coli, three isoenzymes, which are labeled I, II, and III in Fig. 11-3, convert aspartate to (3-aspartyl phosphate, the precursor to the end products threonine, isoleucine, methionine, and lysine. Each product inhibits only one of the isoenzymes as shown in the figure. 

Ziprasidone is oxidatively metabolized by CYP3A4, but it does not inhibit CYP3A4 or other isoenzymes at clinically relevant concentrations. The effect of ketoconazole 400 mg qds for 6 days on the single-dose pharmacokinetics of ziprasidone 40 mg has been evaluated in an open, placebo-controlled, crossover study in healthy volunteers... 

Enzyme activity is regulated in a variety of ways, ranging from controls over the amount of enzyme protein synthesised by the cell or modulation of activity through reversible interaction with metabolic inhibitors and activators or through isoenzymes. 

The effects on bone metabohsm of carbamazepine, valproate, or phenobarbital as monotherapy have been analysed in a case-control study in 118 ambulatory children with epilepsy and corresponding controls (120). Patients taking carbamazepine or phenobarbital had significantly raised alkaline phosphatase and bone and liver isoenzyme activities compared with controls. Although the authors concluded that children who take anticonvulsants may have their bone metabolism affected, this conclusion was based on abnormal values of a surrogate marker for bone disease. 

Unlike oxybutynin IR, oxybutynin XL delivers a controlled amount of oxybutynin chloride continuously throughout the gastrointestinal tract over a 24-hour time period, reducing first-pass metabolism by cytochrome P450 (CYP450) isoenzyme 3 A4, which is... 

In a randomised, placebo-controlled study, 9 healthy subjects were given a single 10-mg nifedipine capsule after a 5-day course of nafcillin 500 mg four times daily. The nifedipine AUC was decreased by 63% and the clearance was increased by 145%, but the effect of these changes on nifedipine pharmacodynamics was not assessed. It was suggested that nafcillin is an inducer of cytochrome P450 isoenzymes, and increased the metabolism of nifedipine. The clinical significance of these changes is unclear. 

Fluvoxamine is an inhibitor of cytochrome P450 isoenzyme CYPl A2, the main isoenzyme involved in the metaholism of tacrine. In vitro study showed that fluvoxamine is a potent inhibitor of tacrine metabolism, and it was therefore predicted that fluvoxamine may dramatically increase tacrine plasma levels in patients. This prediction was confirmed in a placebo-controlled study in 13 healthy subjects who had an eightfold increase in the mean AUC of a single 40-mg dose of tacrine after taking fluvoxamine 100 mg for 6 days. A very large increase in the AUC of the hydroxy-lated metabolites of tacrine, and an eightfold fall in the clearance of tacrine... 

Some NSAIDs are inhibitors of the cytochrome P450 isoenzyme CYP2C9, and inhibit the metabolism of warfarin via this isoenzyme. There is also possibly a pharmacokinetic interaction with NSAIDs that are substrates for CYP2C9. People with variant CYP2C9 (about 5 to 11% of Caucasians) have a lower metabolising capacity for warfarin, and require much lower maintenance doses. It is possible that use of an NSAID that is a CYP2C9 substrate may result in reduced warfarin metabolism, although this requires confirmation in controlled studies. 

A controlled study in 20 healthy subjects found that the pharmacokinetics of a single 300-mg dose of phenytoin were unaltered by zileuton 600 mg every 6 hours for 5 days. An in vitro study found that zileuton had little effect on the isoenzymes responsible for the metabolism of phenytoin. These studies suggest that zileuton is unlikely to affect phenytoin levels in clinical use. 

A study in 9 children with acute lymphoblastic leukaemia or non-Hodg-kin s lymphoma found that the AUC of etoposide and its metabolite etoposide catechol were slightly increased, by 8.6% and 28.4%, respectively, following atovaquone 45 mg/kg daily, when compared with co-trimoxa-zole 150/750 mg/m daily. The mechanism by which this occurs is unclear, but atovaquone may affect the metabolism of etoposide by the cytochrome P450 isoenzyme CYP3A4 or its transport by P-glycoprotein. The authors considered that an interaction with co-trimoxazole was unlikely, so used it as a control, however, ideally this requires confirma-... 

It is likely that nevirapine induces the metabolism of the components of the oral contraceptive by cytochrome P450 isoenzymes. Although it is not known whether these modest reductions in levels would reduce the anti-ovulatory efficacy of the combined oral contraceptive, it would be prudent to assume they could. The manufacturers recommend that combined oral contraceptives and other hormonal methods of birth control should not be used as the sole method of contraception in women taking nevirapine. They suggest that a barrier method (e.g. condoms) should also be used, and note that this is also advisable to reduce the risk of HTV transmission. The Faculty of Family Planning and Reproductive Health Care (FFPRHC) Clinical Effectiveness Unit notes that nevirapine can induce liver enzymes and may reduce the levels of ethinylestradiol and progestogens. They therefore recommend that their guidance on hormonal contraceptives and liver enzyme inducers is followed, see Hormonal contraceptives + Antiepileptics Barbiturates or Phenytoin , p.985 for fiir-ther detail. 

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