* Department of Pharmacology and Physiology, New Jersey Medical School, University of Medicine and Dentistry of New Jersey, Newark, New Jersey 07103, and Schering-Plough Research Institute, Lafayette, New Jersey 07843
1 To whom correspondence should be addressed at Department of Pharmacology and Physiology, New Jersey Medical School, UMDNJ, 185 South Orange Avenue, Newark, NJ 07103. Fax: (973) 972-4554. E-mail: sultatle{at}umdnj.edu.
Received September 10, 2003; accepted November 15, 2003
ABSTRACT
The assessment of the variability of human responses to foreign chemicals is an important step in characterizing the public health risks posed by nontherapeutic hazardous chemicals and the risk of encountering adverse reactions with drugs. Of the many sources of interindividual variability in chemical response identified to date, hereditary factors are some of the least understood. Physiologically based pharmacokinetic modeling linked with Monte Carlo sampling has been shown to be a useful tool for the quantification of interindividual variability in chemical disposition and/or response when applied to biological processes that displayed single genetic polymorphisms. The present study has extended this approach by modeling the complex hereditary control of alcohol dehydrogenase, which includes polygenic control and polymorphisms at two allelic sites, and by assessing the functional significance of this hereditary control on ethanol disposition. The physiologically based pharmacokinetic model for ethanol indicated that peak blood ethanol levels and time-to-peak blood ethanol levels were marginally affected by alcohol dehydrogenase genotypes, with simulated subjects possessing the B2 subunit having slightly lower peak blood ethanol levels and shorter times-to-peak blood levels compared to subjects without the B2 subunit. In contrast, the area under the curve (AUC) of the ethanol blood decay curve was very sensitive to alcohol dehydrogenase genotype, with AUCs from any genotype including the ADH1B2 allele considerably smaller than AUCs from any genotype without the ADH1B2 allele. Furthermore, the AUCs in the ADH1C1/C1 genotype were moderately lower than the AUCs from the corresponding ADH1C2/C2 genotype. Moreover, these simulations demonstrated that interindividual variability of ethanol disposition is affected by alcohol dehydrogenase and that the degree of this variability was a function of the ethanol dose.
Key Words: alcohol dehydrogenase; physiologically based pharmacokinetic modeling; ethanol; risk assessment.
Interindividual variability in therapeutic and toxic responses to xenobiotics results from a variety of pharmacodynamic and pharmacokinetic factors (Clewell and Andersen, 1996; Nebert, 2000
). The assessment of the variability of human responses to foreign chemicals is an important step in characterizing the public health risks posed by nontherapeutic hazardous chemicals and the risk of encountering adverse reactions with drugs. Of the many sources of interindividual variability in chemical response identified to date, hereditary factors are some of the least understood. Much uncertainty is often associated with the potential functional significance of both polygenic control and genetic polymorphisms of proteins that are involved in mediating the actions and/or disposition of foreign chemicals. This in turn leads to uncertainty when attempting to quantify risk associated with certain chemical exposures. New approaches are needed to investigate the potential functional significance of polygenic traits and genetic polymorphisms and to use this information for assessing the consequent interindividual variability in chemical response. Physiologically based pharmacokinetic modeling is ideally suited for the consideration of hereditary factors that affect the pharmacokinetic disposition of specific chemicals and for the estimation of the interindividual variability stemming from these genetic factors (Clewell and Andersen, 1996
; Gentry et al., 2002
). Our study developed a physiologically based pharmacokinetic model for ethanol that includes the hereditary control of the family of enzymes known as alcohol dehydrogenase (EC 1.1.1.1).
Alcohol dehydrogenases catalyze the reversible oxidation of ethanol to acetaldehyde, with corresponding reduction of NAD+ to NADH (Li et al., 2001; Fig. 1). Alcohol dehydrogenase in humans is a dimer and exists as multiple forms, resulting from its well-characterized polygenic control and genetic polymorphisms (Li et al., 2001
). The nomenclature we used for the various forms of alcohol dehydrogenase and the genes that encode these forms were from Duester et al. (1999)
. This enzyme in humans is encoded by at least seven separate genes, and multiplicity of the enzyme exists at three different levels (Li et al., 2001
). Firstly, separate classes of alcohol dehydrogenase are derived from distant gene duplication at early vertebrate times (Jornvall et al., 2000
). Secondly, more recent gene duplications have led to isozyme multiplicity within certain classes (Jornvall et al., 2000
). Thirdly, allelic variants exist within several different isozyme forms (Jornvall et al., 2000
). Those hepatic forms of alcohol dehydrogenase that are primarily involved in the metabolism of ethanol are referred to as class 1. The subunits (monomers) that constitute class 1 alcohol dehydrogenases are encoded by the three genes ADH1A, ADH1B, and ADH1C, giving rise to A, B, and C subunits, which can then combine to form homodimers and heterodimers (Duester et al., 1999
). The ADH1A, ADH1B, and ADH1C subunits share greater than 93% sequence homology (Plapp et al., 1984
). Allelic variants, or genetic polymorphisms, occur at ADH1B to give ADH1B1, ADH1B2, and ADH1B3 subunits and at ADH1C to give ADH1C1 and ADH1C2 subunits. Thus, hepatic class 1 alcohol dehydrogenase in humans can exist as 21 different possible forms (homodimers plus heterodimers), with the specific number and type of forms within an individual dependent on that individual's genotype. Additionally, class 4 alcohol dehydrogenase is a single homodimer and is found primarily in the stomach (Li et al., 2001
).
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MATERIALS AND METHODS
General model structure.
The structure of the physiologically based pharmacokinetic model, minus the metabolism by hepatic alcohol dehydrogenases, was adapted from a previously presented model (Pastino et al., 2000) and is shown in Figure 2. The values for the physiological parameters used, as well as distribution information used for Monte Carlo sampling, are shown in Table 1. The volume of and blood flows to the rapidly perfused and slowly perfused tissue compartments were calculated from the following equations (Pastino et al., 2000
):
![]() | (1) |
![]() | (2) |
![]() | (3) |
![]() | (4) |
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Modeling alcohol dehydrogenases.
Alcohol dehydrogenase isoforms catalyze reversible oxidation of ethanol to acetaldehyde, with corresponding reduction of NAD+ to NADH, by a kinetic mechanism known as the Theorell Chance mechanism (Segel, 1975; Theorell and Chance, 1951
; Fig. 1). To incorporate the genetic control of alcohol dehydrogenase into the physiologically based pharmacokinetic model, a metabolism submodel for each possible form of alcohol dehydrogenase (a total of 21 possible forms and, therefore, 21 submodels) was developed based on the differential and algebraic equations describing the Theorell Chance kinetic mechanism (Fig. 1). The equations for each submodel were structured as follows:
![]() | (5) |
![]() | (6) |
![]() | (7) |
![]() | (8) |
![]() | (9) |
Solving Equations 5-9 required values for the rate constants descriptive of the Theorell Chance mechanism for each enzyme form (Fig. 1). Although these values have not been published, kinetic equilibrium constants (see Segel [1975] for a detailed discussion of these) such as Vmax (maximum velocity), Km (substrate concentration at half of maximum velocity), and Ki (the product inhibition constant) have been published for all homodimers of class 1 alcohol dehydrogenases (Bosron et al., 1993; Yin et al., 1984
), and the appropriate rate constants could be calculated from these equilibrium constants. It should be noted that alcohol dehydrogenase activity runs in both the forward and reverse directions (Bosron et al., 1993
) and, therefore, has potentially four substrates and four products (Fig. 1). For example, when considering the forward reaction, ethanol and NAD+ are substrates with the products being acetaldehyde and NADH. Conversely, with the reverse reaction, the substrates are acetaldehyde and NADH and the products are ethanol and NAD+. Each substrate has a Vmax and Km, while each product has a Ki (a product inhibitory constant; Segel, 1975
). Those rate constants required for the Theorell Chance mechanism were calculated from the published kinetic equilibrium constants using a series of equations, derived in Segel (1975)
, as follows:
![]() | (10) |
![]() | (11) |
![]() | (12) |
![]() | (13) |
![]() | (14) |
![]() | (15) |
![]() | (16) |
![]() | (17) |
Rate constants in Equations 10-17 are identical to those described in Figure 1. The Theorell Chance kinetic mechanism was modeled with rate constants (Fig. 1) rather than equilibrium constants due to the extreme complexity of the velocity equations containing the equilibrium constants (Segel, 1975), which rendered them impractical for use in our model. Furthermore, it should be noted that the published Vmax values for isoforms could not be used directly since recovery data for the purified forms were not available. All rate constants (Table 2) were calculated directly from published kinetic equilibrium constants reported by Bosron et al. (1983)
, and Yin et al. (1984)
, using the above equations, except for k1 for the AA homodimer, k3 for the B1B1 homodimer, and k2 for both the C1C1 and C2C2 homodimers, which were determined as described below.
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![]() | (18) |
For the B1B1 homodimer, the equation describing the relationship between k3 and ethanol concentration was as follows:
![]() | (19) |
The equation describing the relationship between C1C1 and C2C2 and their k2 values (an inverse of the Hill equation; Segel, 1975) was as follows:
![]() | (20) |
Equations 1820 were incorporated into the various alcohol dehydrogenase submodels to account for the substrate inhibition and negative cooperativity.
Class 1 alcohol dehydrogenase isoforms can exist as homodimers and heterodimers. The roles of subunits or monomers in homodimer activity have been studies over the past two decades. While early investigations reported that alcohol dehydrogenase monomers were enzymatically inactive (Anderson and Mosbach, 1979 Briganti et al., 1989
), Ehrig et al. (1993)
, using an extremely high ethanol concentration (1.5 M), demonstrated activity with the B1 monomer that exceeded homodimer activity. They noted two important differences between monomers and homodimers: (1) three amino acids of the outer part of the hydrophobic ethanol-binding barrel of a subunit are furnished by the second subunit, leading to a decreased affinity of the monomer for ethanol; (2) an incomplete coenzyme-binding site in a monomer leads to an increased rate of dissociation of NADH, increasing the specific activity of the monomer. Consequently, higher ethanol concentrations were required to drive the reaction with the monomers compared to the homodimers, but the monomer reaction proceeded at a faster rate once it was underway (Ehrig et al., 1993
). Thus, monomers possess enzymatic activity towards ethanol albeit atypical.
Given that the dimerization of subunits is necessary to obtain typical alcohol dehydrogenase activity, an important issue is whether or not the subunits of homodimers interact differently when present in heterodimers. That is, are the activities of heterodimers predictable based on knowledge of the homodimer activity? Extensive kinetic information has been published for the homodimers but not for the heterodimers. Although Mardh et al. (1986) stated that the turnover rates of human AB1, B1C1, and B1C2 isozymes were considerably faster than would be predicted from the values of the relevant homodimers, the evidence to support this statement is not compelling. Wagner et al. (1983)
, who were quoted by Mardh et al. (1986)
in support of their conclusion, examined turnover numbers of certain heterodimers and homodimers. Of the heterodimers examined, only the B1C2 form had a turnover number that was not between the two respective homodimers. Furthermore, the authors stated that, in successive liver preparations, turnover values were found to vary no more than 3-fold (Wagner et al., 1983
), indicating that B1C2 heterodimer activity could have been between that of B1B1 and C2C2.
Yin et al. (1984) have reported that the Vmax and Km values for ethanol metabolism by B1B2, AB2, and B2C1 were intermediate between the respective homodimers. Likewise, Fong and Keung (1987) reported that AB2, B2C2, and B2C1 heterodimers had activity that can be predicted from the individual subunits, although the substrate was cyclohexanol and not ethanol. Therefore, for the current ethanol model it was assumed that heterodimer activity could be predicted from the activity of each subunit. In other words, each of the two active sites contained within a heterodimer was assumed to act independently as a monomer. Hence, the concentration of each active site of a heterodimer was equal to one-half of the heterodimer concentration. The differential equations (based on the Theorell Chance kinetic mechanism) for a heterodimer metabolism submodel, using the AB2 heterodimer as an example, are as follows:
![]() | (21) |
![]() | (22) |
![]() | (23) |
![]() | (24) |
The algebraic expressions describing the amount of A monomer and B2 monomer present during the metabolism of ethanol are represented as the following equations:
![]() | (25) |
![]() | (26) |
Solution of equations 21-26 for the AB2 heterodimer resulted in a rate of metabolism in vitro of the AB2 heterodimer that was midway between the rates of metabolism in vitro of the AA and B2B2 homodimers (Fig. 3). All additional heterodimers were modeled in a similar fashion, and activities of heterodimer submodels were midway between that of the respective homodimers (data not shown), indicating that the heterodimer submodels functioned as intended. Finally, it was assumed that the levels of various isoforms within a specific individual were expressed equally. No published information is available regarding expression levels of alcohol dehydrogenase isoforms.
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Conducting simulations.
All simulations were conducted using Advanced Continuous Simulation Language (ACSL, AEgis Technologies Group, Inc., Huntsville, AL) on a PC. Monte Carlo simulations were performed in ACSL Math.
RESULTS
Incorporation of genetic control of alcohol dehydrogenase activity into a previously developed physiologically based pharmacokinetic model for ethanol (Pastino et al., 2000) was based on inclusion of metabolism submodels for each possible enzyme isoform, which were then "switched on" or "switched off" as a function of the genotype simulated. The submodels were constructed based on rate constants descriptive of the Theorell Chance mechanism for each enzyme form (Table 1 and Fig. 1). Figure 4 shows examples of four metabolism submodels accurately simulating in vitro metabolism of ethanol by those homodimers reported to display substrate inhibition or negative cooperativity. These four homodimers are the only homodimers for which primary data from in vitro incubations have been published.
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Empirical determination of the total alcohol dehydrogenase active site concentration (ET in Equation 8) within liver has not been previously reported. Therefore, optimization of the model to three data sets from subjects of known genotypes (Thomasson et al., 1995) was used to estimate this important parameter (Fig. 5). It should be noted that Thomasson et al. (1995)
presented only one representative set of blood ethanol decay data. However, Thomasson et al. (1995)
also presented mean values for pharmacokinetic parameters in each genotype (such as dose, y intercept, and zero-order rate constant for elimination) that allowed the calculation of the mean blood ethanol levels at the time points at which blood was sampled from the subjects. These mean blood ethanol levels are the data points shown in Figure 5. Since all three genotypes had similar active site concentrations (Fig. 5), the mean value (0.03 mM) was selected for use in the model.
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The rate of ethanol metabolism in vivo relates to the amount of alcohol dehydrogenase present; the functional characteristics of the isoforms present; and the concentrations of NAD+, NADH, and acetaldehyde (Li et al., 2001). Since acetaldehyde is a strong inhibitor of ethanol metabolism, the total amount of aldehyde dehydrogenase and the functional characteristics of the isoforms present might modulate the rate of ethanol metabolism by alcohol dehydrogenase (Bosron et al., 1993
; Eriksson et al., 2001
; Li et al., 2001
). However, hepatic aldehyde dehydrogenase activity is likely very high, with the exception of ALDH2*2 since blood acetaldehyde levels following ethanol administration are usually reported as a very small fraction of the blood ethanol and acetate levels (Li et al., 2001
). The ALDH2*2 allele encodes for a physiologically inactive aldehyde dehydrogenase that might lead to some acetaldehyde accumulation following ethanol exposure (Borras et al., 2000
), although there is no direct evidence for this assertion. The ALDH2*2 allele is very rare in non-Asian populations. In our model, significant simulated accumulation of NADH and/or acetaldehyde slowed the rate of ethanol metabolism (data not shown). Therefore, to focus exclusively on the role of alcohol dehydrogenase genetics on ethanol disposition, NAD+ was set at a constant physiological level, while NADH and acetaldehyde were reduced to concentrations that did not result in significant product inhibition. Though minimizing hepatic NADH and acetaldehyde concentrations could have represented an oversimplification, any limited product inhibition that might occur in vivo with alcohol dehydrogenase and that is unaccounted for in the ethanol model would be compensated for by a slightly lower model enzyme active site concentration (since enzyme active site concentration was determined by optimization).
As noted by Thomasson et al. (1995), Bosron and Li (1986)
reported that the Vmax for ethanol for the B2B2 and B3B3 homodimers greatly exceeded that of the B1B1 homodimer. For example, the B2B2 homodimer Vmax was about 44 times greater than the B1B1 Vmax (Bosron and Li, 1986
). In our study, while the presence of a B2 subunit reduced the ethanol AUC by increasing ethanol metabolism (Fig. 10), this AUC reduction was not as great as the Vmax differences documented in Bosron and Li (1986)
. However, Bosron and Li (1986)
compared activities of purified homodimers in vitro under optimal conditions, whereas the present study compared AUCs between simulated subjects of different genotypes, where each individual could have six to 15 isoforms of varying activity present depending on that individual's genotype. Therefore, it is not surprising that the comparison of in vitro activities of individual purified homodimers under ideal conditions show greater differences than do the ethanol-metabolizing capacity of simulated subjects of different genotypes.
Many drug-metabolizing enzymes and drug transporters have been shown to exist in multiple forms as a result of genetic polymorphisms and/or polygenic control (Nebert, 1999). However, the functional significance of this hereditary control is often difficult to assess without the existence of extremely discordant phenotypes (Nebert, 2000
). Biologically based modeling can play an important role in assessing the importance of hereditary control of chemical disposition and/or chemical response in the absence of extremely discordant phenotypes. In the case of alcohol dehydrogenase, some differences in phenotype might be expected as a result of the differences in activities between the isoforms encoded by ADH1 genes (Borras et al., 2000
; Goedde et al., 1992). Yet, as described by Borras et al. (2000)
, two reports failed to find differences in the ß60 values (slopes of the pseudo-linear portion of the ethanol blood decay curves) of ethanol between east Asian individuals with the ADH1B1 and ADH1B2 alleles (Mizoi et al., 1994
; Yamamoto et al., 1993). However, the modeling results we have presented indicate that the selection of the pharmacokinetic parameter(s) evaluated is important in the comparison of ethanol disposition in subjects of different genotypes (Figs. 8-10). Neither Yamamoto et al. (1993) nor Mizoi et al. (1994)
evaluated blood ethanol AUC, the pharmacokinetic parameter that displayed the most dependence on genotype (Figs. 8- 10). Furthermore, simulations of the Mizoi et al. (1994)
experiments (Fig. 7) suggested that genotype-dependent differences in ethanol pharmacokinetic disposition could have been obscured since the ADH1C genotypes were not reported and since ß60 might not be the optimal pharmacokinetic parameter to analyze. The simulations shown in Figure 7 indicate that ß60, a parameter often used to describe ethanol pharmacokinetic disposition, was only slightly to moderately affected by alcohol dehydrogenase genotype.
As discussed by Haber et al. (2002) and Gentry et al. (2002)
, safe or allowable exposure levels to environmental toxicants are often established by the identification of a critical effect level, which is then lowered by divisions with uncertainty factors to account for certain extrapolations and deficiencies in data (Gentry et al., 2002
). In recent years, efforts have been made to refine the selection of more appropriate uncertainty factors. For example, separation of intraspecies uncertainty into toxicokinetic and toxicodynamic components to form chemical-specific adjustment factors allows the introduction of adjustment factors that are based on available toxicokinetic and/or toxicodynamic data (Gentry et al., 2002
). As such, physiologically based pharmacokinetic modeling linked with Monte Carlo sampling has been shown to be a useful tool for quantification of interindividual variability in chemical disposition and/or response when applied to biological processes that displayed single genetic polymorphisms (Barton et al., 1996
; El-Masri et al., 1999
; Gentry et al., 2002
; Haber et al., 2002
). We have extended this approach by modeling the complex hereditary control of alcohol dehydrogenase, which includes polygenic control and polymorphisms at two alleles, and by assessing the functional significance of this hereditary control on ethanol disposition. Two observations are noteworthy. Firstly, pharmacokinetic variability assessed by peak blood ethanol concentration and time-to-peak blood levels was substantially lower than variability assessed by AUC (Figs. 810). Consequently, considerably different data-derived uncertainty factors or chemical-specific adjustment factors could be calculated, depending on which pharmacokinetic parameter is chosen. Secondly, the variability of blood ethanol AUC increased in a manner disproportionate to dose, probably due to mixed-order or pseudo-zero-order metabolism. Therefore, different chemical-specific adjustment factors could be derived from the pharmacokinetic disposition of ethanol at different doses.
An important objective of the present study was to identify the kinds of data that might assist in the incorporation of complex hereditary control of chemical disposition into physiologically based pharmacokinetic models. Those kinds of data that applied to modeling genetic control of alcohol dehydrogenase and that could be extended to other biotransformation enzymes under complex genetic control are listed in Table 3, including some significant data gaps that were identified. With this report, expression levels of alcohol dehydrogenase forms would have eliminated the need for optimization to obtain enzyme active site concentration (Fig. 5), while additional studies documenting ethanol disposition in subjects of known genotypes would have allowed improved model validation. Undoubtedly, the extensive kinetic analyses of purified human alcohol dehydrogenase homodimers (Bosron et al., 1983; Yin et al., 1984
) were key in the design of the metabolic submodels for each isoform. This is somewhat at odds with Haber et al. (2002)
, who more strongly endorsed the use of enzyme activities from tissues of individuals having known genotypes rather than activities from purified variant proteins.
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The limited number of published studies documenting the pharmacokinetic disposition of ethanol in subjects of known genotype represents a significant data gap (Table 3). While the modeling results presented for this study suggest that alcohol dehydrogenase genotype has a significant impact on ethanol pharmacokinetic disposition (Fig. 10), the functional significance of the genetic control of alcohol dehydrogenase remains an unresolved issue.
ACKNOWLEDGMENTS
This work was funded by a research grant from the American Chemistry Council. All model code will be made available upon request to the authors.
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