The removal of ethanol in the liver involves its oxidation to acetaldehyde by nicotinamide...

The removal of ethanol in the liver involves its oxidation to acetaldehyde by nicotinamide adenine dinucleotide (NAD+) catalyzed by the enzyme liver alcohol dehydrogenase (LADH). The overall reaction is

The reaction follows a sequential or ordered mechanism wherein NAD+ must bind to the enzyme before C2H5OH binds to form a ternary complex, and CH3CHO and H+ dissociate from the ternary complex before the NADH is released.

a. Write a detailed set of reactions involving intermediate complexes so as to represent the mechanism of this reaction. Indicate each step as being reversible and designate the rate coefficients (in order) by k1 k2, etc., for forward reaction steps and k-1 k-2, etc., for reverse steps.

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b. Give a plausible explanation for the observation that the removal of ethanol from the body obeys zero-order kinetics.

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c. The rate-limiting step for the overall reaction under physiological conditions and in the presence of an intoxicating level of ethanol is the final dissociation of the LADH-NADH complex. The rate coefficient for this step has been measured to be 3.1 s-1. Using appropriate data given in problem, calculate the amount (in mmol) of the LADH enzyme present in the liver,

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Alcoholism in humans is a disorder that has serious sociological as well as biochemical consequences. Much research is currently under way to discover the nature of the biochemical consequences, to understand better how this disorder can be treated. You can analyze this problem using a variety of kinetic approaches.

Problem

Alcohol taken orally is transferred from the gastrointestinal tract (stomach, etc.) to the bloodstream by a first- order process with t1/2 = 4 min. (These and subsequent figures are subject to individual variations of ±25%.) The transport by the bloodstream to various aqueous body fluids is very rapid; thus, the ethanol becomes rapidly distributed throughout the approximately 40 L of aqueous fluids of an adult human. These fluids behave roughly like a sponge from which the alcohol must be removed.

The removal occurs in the liver, where the alcohol is oxidized in a process that follows zero-order kinetics. A typical value for this rate of removal is about 10 mL ethanol h-1, or 4 X 10-3 mol (L of body fluid)-1 h-1. Consumption of about 1 mol (46 g, or 60 mL) of ethanol produces a state defined as legally intoxicated. (This amount of alcohol is contained in about 4.5 oz of 80-proof liquor.) At this level, alcohol is rapidly taken up by the body fluids and only slowly removed by the liver. In the liver, ethanol (C2H5OH) is oxidized to acetalde-hyde in the presence of the enzyme liver alcohol dehydrogenase (LADH). The overall process may be represented by the equation

d. Relaxation measurements using the temperature-jump technique were applied to the binding of NAD+ at two different concentrations, to LADH in the absence of ethanol. The relaxation times measured were

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where [LADH] ≪ [NAD + ]. Use these relaxation times to calculate the coefficients k1 and k-1 for the reactions

e. By contrast with ethanol, other simple alcohols, such as methanol (CH3OH) and ethylene glycol (HOCH2CH2OH) are highly toxic to humans. Methanol is oxidized to formaldehyde, which reacts irreversibly with proteins (it is a commonly used cross-linking agent, or fixative). Ethylene glycol is not toxic, but its oxidation product oxalic acid is. Each alcohol is about as good as ethanol as a substrate for LADH. A common antidote for methanol or ethylene glycol poisoning is to administer ethanol in large quantities. Propose a rationale for this therapy.