Michaelis constant Microsomal

By means of an apparent Michaelis constant (A mapp) together with a maximum rate ( max) of butadiene metabolism both obtained with human liver microsomes (Filser et al., 1992), Filser et al. (1993) constructed a human model which was later extended by Csanady et al. (1996) for the butadiene metabolites epoxybutene and diepoxybutane. For butadiene and epoxybutane, the required human tissue air partition coefficients were measured using autopsy material (Table 23). Filser et al. (1993) investigated the influence of styrene co-exposure on butadiene metabolism by assuming competitive interaction. Simulations for a 70-kg man exposed over 8 h to 5 or 15 ppm [11 or 33 mg/m3] butadiene indicated total amounts of butadiene metabolized of 0.095 and 0.285 mmol, respectively, reduced by about 19% and 37% as a result of co-exposure to 20 and 50 ppm styrene, respectively. No influence of butadiene on styrene metabolism was noted. 

Based on in vitro studies of hepatic microsomal-catalysed oxidative reactions, using liver specimens from a variety of animal species including man, and marker substrates for the various reactions, it can be concluded that neither the measured levels of cytochrome P450 and cytochrome nor the activity of NADPH-cytochrome c reductase accounts for species variations in the capacity of oxidative reactions (McManus and Ilett, 1976 Dalvi et al., 1987 Souhaili-El-Amri et al., 1986). However, species variations could be attributed to differences in values of the kinetic parameters (Michaelis constant, Km, and the maximum reaction velocity, Vmax) associated with individual reactions. 

The mechanism of oxidation of decane has been studiedusing [l- C]decane. The oxidation of decane to decanol, decanoic acid and decamethylene glycol by mouse liver microsomes required NADPH and O2. This indicates that the process is initiated by the hydroxylation of decane to decanol. The Michaelis constant for this reaction was found to be 0.5 mM and the optimum pH was about 8.1. The reaction has been strongly inhibited by CO and t-butyl isocyanide. Evidence has been presented indicating that the enzymic systems for decane hydroxylation and for decanoate co-oxidation are not identical. Organometallic oxidation catalysts have been reviewed by Speier " ... 

Phenobarbital produces a relatively uncomplicated effect on the kinetics of microsomal drug metabolism, namely an increase in (the maximal velocity) with no significant effect on apparent (Michaelis constant). 

The gas-uptake data for rats were well described using a single Michaelis-Menten equation to describe metabolism. For the mouse inhalation studies, a simple Michaelis-Menten equation failed to adequately describe the chloroform-metabolizing capacity based on the data collected and model constants. The authors suspected that, following the administration of chloroform (particularly at higher concentrations), destmction of microsomal enzymes and subsequent resynthesis of microsomal enzymes was important in the mouse. This phenomenon has been documented in phenobarbital-induced but not naive rats. To account for this phenomenon, a first-order rate constant for the loss and subsequent regeneration of metabolic capacity was incorporated into the model for mice only. 

With UDP-glucuronic acid as the variable substrate and bilirubin at constant concentration, Michaelis-Menten kinetics were obeyed (A2, H2, HIO, P5, V2, W12). Rat liver microsomal preparations treated in different ways yielded apparent Km for UDP-glucuronic acid of 0.37-0.70 mM (A2, H2, HIO, V2), with albumin-bound bilirubin as the aglycon a higher value (1.66 mAf) was found in a carrier-free system (W12). These values probably do not represent Km for UDP-glucuronic acid at saturation with bilirubin (P5, V6). Under certain conditions of activation, cell extracts from livers of newborn and adult rats, wdicn tested with o-amino-... 

Microsomes were prepared from rats. The Michaelis-Menten constant of highly purified enzyme did not differ from the apparent Km of the enzyme in microsomes. Microsomes also supported formation of norbenzydamine as a result of the presence of cytochrome P450. 

Table III. Apparent Michaelis—Menten Aflinity Constants of Aminopyrine N-Demethylation in Guinea Pig Microsomes under Varying Intakes of Ascorbic Acid...

Using liver microsomes from different species, the intrinsic clearance (Cl nt) for each species can be determined and then scaled to hepatic clearance. This is typically done by first determining in vitro Km (the Michaelis-Menten constant) and Vmax (the... 

Youdim K, DodiaR (2010) Comparison between recombinant P450s and human liver microsomes in the determination of cytochrome P450 Michaelis-Menten constants. Xenobiotica 40 235-244... 

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