Vmax values

The evaluation of the KM and vmax values for enzymatic reactions is usually carried out using a linear plot of Eq. 14. The Lineweaver-Burke equation is widely used to this aim ... 

In liver microsomes, the formation of 1-, 2-, and 3-hexanol from -hexane was best described kinetically by a 2-enzyme system, while for lung microsomes, single-enzyme kinetics were indicated for each metabolite. For conversion to 1-hexanol, apparent Km values were 0.4 and 300 M, and Vmax values were 0.09 and 1.2 nmol/mg protein/min, respectively. For conversion to 2-hexanol, apparent Km values were 6 and 1,100 M, and Vmax values were 1 and 4.6 nmol/mg protein/min respectively. Insufficient information was available to estimate the high-affinity activity for 3-hexanol, the low-affinity activity had an apparent Km of 290 M and a Vmax of 0.5 nmol/mg protein/min. In the lung, Km values were 9,... 

In terms of kinetic parameters, reversible inhibitors act to alter the rate of certain enzyme-catalysed reactions by changing the Km or Vmax values (designated ] warent Qr K m and /illIl ll silrcnL or respectively to distinguish these changed values from the real values. (See Chapter 2 for a more detailed account.)... 

Two examples of aryl esters are given in Table 8.5, namely the 4-chloro-phenyl and 4-nitrophenyl esters of nicotinic acid (8.33). Under physiological conditions of pH and temperature, these two compounds were clearly much more susceptible to chemical hydrolysis than the alkyl and arylalkyl esters in Table 8.5. Their affinity for carboxylesterase and human plasma hydrolases, as assessed by the Michaelis constant Km, was generally higher, while nothing can be said regarding Vmax values. 

Like in Chapt. 7, we begin the discussion with acetates, since acetic acid is the simplest nontoxic acyl group, formic acid being less innocuous. An informative study was carried out to compare the kinetics of hydrolysis of two types of corticosteroid esters, namely methyl steroid-21-oates (which are active per se) and acetyl steroid-21-ols (which are prodrugs), as exemplified by methyl prednisolonate (8.69) and prednisolone-21-acetate (8.70), respectively [89]. In the presence of rat liver microsomes, the rate of hydrolytic inactivation of methyl steroid-21-oates was much slower than the rate of hydrolytic activation of acetyl steroid-21-ols. Thus, while the Km values were ca. 0.1 -0.3 mM for all substrates, the acetic acid ester prodrugs and the methyl ester drugs had Vmax values of ca. 20 and 0.15 nmol min-1 mg-1, respectively. It can be postulated that the observed rates of hydrolysis were determined by the acyl moiety, in other words by the liberation of the carboxylic acid from the acyl-enzyme intermediate (see Chapt. 3). 

With knowledge of iCi, a and p for a reversible inhibitor, along with Km and Vmax values for the substrate of interest, it is possible to fit hyperbolic curves to multiple data sets by applying the appropriate inhibitor equation. It is a relatively straightforward task to enter such equations into graphing programs such as... 

Medium chain acyl-CoA dehydrogenase deficiency apparent Km and Vmax values for fibroblast acyl-CoA dehydrogenase towards octanoyl CoA in patient and control cell lines. 

If such enzymes occur at the same levels in relevant microbial populations, Vmax may be directly related to other metrics of biomass presence such as cell numbers, biomass dry weight, or protein concentrations. In an attempt to enable extending results from one system to another (e.g., from laboratory observations to field situations), one often normalizes Fmax by such biomass parameters. For example, in Table 17.7, the observed Vmax values are normalized to the protein contents of the tested microbial populations or isolated enzymes, and the result is given as values Vmax (the prime is added to emphasize the normalization). To apply such information to new situations, one must multiply the normalized maximum velocities by a measure of the relevant enzyme concentration or biomass protein in the new system of interest (e.g., Vmax x microbial protein content in new case involving intact microorganisms). Of course, one is assuming that the ratio of enzyme to total protein is the same in the old and new situation. 

What factor must be used to convert a VW value determined using a pure culture of a particular microbial species and given in mol substrate kg-1 protein - s 1 to a Vmax value in units of mol substrate L s What does that factor represent ... 

Some enzymes are able to differentiate between D and L isomers of substrates. Dopa, with a chiral center, exists in enantiomeric pairs, D and L. The naturally occurring dopa molecule has the L configuration, so it might be expected that the enzyme-catalyzed oxidation of L-dopa is more facile than that of D-dopa. The relative reactivity can be determined in a quantitative way by comparing Ku or Vmax values. Does tyrosinase exhibit stereoselectivity ... 

Some kinetic constants for several substrates are given in Table III Km values for each substrate are roughly comparable for the enzyme from each organism, and Vmax values indicate the degree of reactivity for each substrate. Several kinetic studies have provided evidence that... 

Nordlie and Lygre (118, 132) have carried out detailed pH kinetic studies of PPi-glucose phosphotransferase, CDP-glucose phosphotransferase, and glucose-6-P phosphohydrolase activities with partially purified rat liver enzyme preparations, and they have concluded, on the basis of sharp, unit-changes in slopes of Dixon (133) plots of pKm for phosphate substrates vs. pH near 6.5, that enzyme-bound histidine may participate directly in the catalytic process. Values of Km for glucose were unaffected by reaction mixture pH Km values and relative Vmax values calculated for infinite concentrations of all substrates are tabulated in Table IX. 

---