Liver velocity constants

Some velocity constants, as well as dissociation constants for various enzyme — substrate complexes, have been obtained by a comprehensive study of the action of rabbit-liver phosphorylase. ... 

The results of stopped-flow studies of the lactate dehydrogenase reaction have proved to be more difficult to interpret than those of alcohol dehydrogenase. The dissociation velocity constants for the binary NADH compounds of the pig heart and skeletal muscle lactate dehydrogenases are much larger than that for liver alcohol dehydrogenase, and also larger than the maximum specific rates of lactate oxidation at pH 6.0-7.0 (Table VII). Some earlier step must therefore be rate-limiting. 

The kinetics of the liver enzyme is quite other than that of the yeast enzyme. Here we have to do with a simpler reaction process which can be expressed with only three equations and six velocity constants. 

Male Fischer 344/N rats were exposed via the nose only for 6 h to concentrations of vinylidene fluoride ranging from 27 to 16 000 ppm [71-42 000 mg/m. Tidal volume (mean, 1.51 mL/brcath) and respiratory frequency (mean, 132 breaths/min) were not influenced by exposure concentration. Steady-state blood levels of vinylidene fluoride increased linearly with increasing exposure concentration up to 16 000 ppm. Vinylidene fluoride tissue/air partition coefficients were determined experimentally to be 0.07, 0.18, 0.8,10, and 0.29 for water, blood, liver, fat and muscle, respectively. Previously published detenninations (Filser Bolt, 1979) for the maximum velocity of metabolism in mg/li/kg) and Michaelis Menten constant (K in mg/L) are 0.07 and 0.13, respectively. Time to reach steady-state blood levels of vinylidene fluoride was less than 15 min for all concentrations. After cessation of exposure, blood levels of vinylidene fluoride decreased to 10% of steady-state levels within 1 h. Simulation of the metabolism of vinylidene fluoride mdicated that although blood levels of vinylidene fluoride increased linearly with increasing exposure concentration, the amount of vinylidene fluoride metabolized per 6-h exposure period approached a maximum at about 2000 ppm [5240 mg/m vinylidene fluoride (Medinsky et al., 1988). 

Classification The classification of the hepatobiliary enzymes essential for enzyme diagnostics is based on their characteristic nature - i. e. excretory, secretory and indicator enzymes, (s. tab. 5.5) They are located predominantly within the liver cells and the biliary ducts as well as within the hepatic lobules. The speed of enzyme elimination does not depend on the blood enzyme levels, but follows an exponential curve. This allows the computation of the half-life of enzymes within the plasma, which is not influenced either by gender or age and is a typical enzyme characteristic. The velocity of enzyme elimination is largely constant, (s. tab. 5.5) However, in chronic diseases of the liver, it is known, for example, that GPT is usually eliminated faster than GOT despite its longer half-life. 

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. 

Fig. 2. Reaction of 3 -p-fluorosulfonylbenzoyladenosine with bovine liver glutamate dehydrogenase. Glutamate dehydrogenase (021 mg/ml) was incubated with 3 -FSBA (0.496 mil/) at 24° in 0.01 M sodium barbital buffer (pH 8) containing 0.43 M KCl and 5% ethanol. At each indicated time, an aliquot was withdrawn, diluted 20-fold with Tris-0.1 M acetate buffer (pH 8) at 0°, and assayed (A) in the absence and (B) in the presence of 100 yM ADP. Inset Determination of the pseudo first-order rate constant from the decrease in activation by ADP. (Ft and Fo are the enzymic velocities measured in the presence of ADP and the given and zero time, respectively, and F > is the constant velocity at the end of the reaction. The pseudo first-order rate constant calculated is 0D351 min. ) Data are taken from P. K. Pal, W. J. Wechter, and R. F. Colman, Biochemistry 14, 707 (1975).

The oxidation rate was also dependent on the concentration of hypotaurine in the incubation medium. The reaction appeared to obey simple Michaelis-Menten kinetics (Fig. 2). The kinetic constants were estimated from a linear transformation of the Michaelis-Menten equation in a t against v/s plot. The apparent Michaelis constant ( ) was about 0.2 ramol/1 and the maximal velocity (F) about 0.1 ymol/s X kg. In our crude liver homogenate the apparent for hypotaurine oxidation was of the same order of magnitude as for the partially purified L-cysteine sulphinate decarboxylase (Jacobsen et al., 1964), the preceding enz3nne in the biosynthesis pathway. 

Figure 1. PBPK model for dX -trans-veimoic acid and its metabolites. Abbreviations are Dy intravenous dose (mg) Q or P or C, flow rate or partition coefficient or concentration (mg/1) subscripts c (cardiac), f (fat), g (gut), 1 (liver), pi (placenta), r (richly perfused tissues muscle, bone), s (slowly perfused tissues mammary gland, uterus), sk (skin) Dsc diffusivity in stratum comeum (cm%r) k rate constant subscripts b (biliary clearance), CO2 (side chain oxidation to carbon dioxide/hr), ct (cis/trans isomerization), tc (trans/cis isomerization) e (diffusion limited transfer between placenta and embryo, 1/hr) f (fecal excretion/hr) h (hydrolysis of glucuronide/hr) o (oral absorption) r (intestinal absorption) v (intravenous injection) K or V (affinity constant or maximum velocity where mg = apparent Michaelis-Menten glucuronidation constant and mx = apparent Michaelis-Menten oxidation constant). Rounded box indicates the submodels as diagrammed in Figures 2-5. (Reproduced with permission of Mosby-Year Book, Inc. from the American Academy of Dermatology. Clewell, [29].)...

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