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1) Many different experimental systems - from isolated or recombinant proteins, subcellular fractions, cells, etc. all the way up to animals and animal populations - are used to study detoxication and the toxic/genotoxic effects of xenobiotics. Which types of questions concerning biochemical toxicology can be answered using these different systems? Describe the experimental strengths and weaknesses of these different systems. (ANSWER = 1 A4 page, 10 points)

Ans: The majority of experiments studies are focused to detoxification of the cell and to determine what types of toxic/genotoxic effects caused by xenobiotics. Xenobiotic metabolism is a common or general protective mechanism to against toxic compounds. In general toxic compounds are modified by xenobiotic metabolism and to neutralize their toxicity amd to facilitate their excretion. In vitro studies are currently most used analysis to studies of xenobiotic effects. Most toxicologists believes in vitro toxicology tests can be time saving and cost reducing and mostly no objection from society. The following points are concerned to precede an experiment.

  • Hazard identification: this focuses, to determine substances(xenobiotic)intrinsic toxicity (i.e birth defects, cancer,eye irritation etc)
  • Many environmental chemicals are toxic by their ability to interact with specific receptors.To focuse on How they interact with receptors.
  • In vitro studies,commonly used to determine genotoxic potential and environmental agents, here question is that the effect of such agents on genetic end points in vivo.
  • Drug safety assesement
  • Blood, bone marrow,thymus,spleen,lymph nodes are most complicated system of tissues. these cells are modulated directly or indirectly by xenobiotics.

Strengths and weaknesses:

Invitro analysis is more short term( probably 4 years) and cost cost reducing method instead of invivo

2) Different organs and even different cell types within one and the same organ differ in their susceptibilities to the toxic and genotoxic effects of xenobiotics. Discuss the major factors which contribute to such differences in susceptibility, e.g., pharmacokinetics, differences in xenobiotic metabolism and DNA repair, hormonal polymorphism, etc. (ANSWER = 1 A4 page, 10 points)

Ans: the major factors are contributed to toxic of a cell that are divided into two categories.i.e, chemical factors and biological factors. Tissue and organ specificity is included in biological factors.

Toxic and genotoxic effect of xenobitics depending upon characteristics of particular tissues or organ of the species. for instance,the chemical compound carcinogen diethylstilbestrol can cause tumours when exposed to female estrogen like mammalian glands,uterus and vagin. But in males,this same compound can cause kidney tumours.so many factors are included in this mechanism.Such as pharmacokinetics, xenobiotic metabolism and DNA repair, hormonal polymorphism.

Pharmacokinetics; absorption from the site of exposure, distribution in the tissues and biotransformation.these processes are mainly exist in drug interaction with in the cell.there are many mechanisms are involved in the process,these may involve blockade of receptor or alteration of metabolism or chemical complexation.the interaction of the toxic compounds at the site of action (receptor),it may be reversible or irreversible.

Xenobitic metabolism: enzyme inhibition is basic parameter to stop or alter the toxicity. It may depending up on the structure and physiological properties of compound and the enzyme available in the exposed tissue.partition coefficient,stereochemistry of a comound and the functional groups present on a molecular compound. These all factors may influence the particular metabolic transformation.

Xenobiotics can be regulated by hormones present in particular organ or tissues.some types of tissues have more concentration of hormones that can be effected to toxicity of the cell.for example, CYP17 and SRD542 polymorphisms could influence to prostrate cancer, which are located on 10q24 chromosome and encoded p450c17 that plays an important role in testosterone biosynthesis.

3) Predict how the following compound would be metabolized in mammalian cells. Indicate which enzymes catalyze the various reactions. What kinds of reactive metabolites might be produced? Would the metabolites formed in hepatocytes be excreted in the urine or bile? Why? (ANSWER = 1 A4 page, 10 points)

4) The figure below illustrates a so-called threshold effect of a xenobiotic. Describe at least 4 different principle types of mechanisms which could lead to such an effect. (ANSWER = 1 A4 page, 10 points)

Ans: threshold effect of xenobiotic is not shows detactable response in the patients. It is named as “No observed effect level” (NOEL). This type of condition may leads to eighther quantal response such as death or pathological lesions or enzyme inhibition or binding to a endogenous receptor.the main reason to cause of threshold it depending on saturation of an enzymen or physiological repair system of enzyme.

For example, nitric oxide (NO•): when its low concencentration acts as a signaling molecule with regulatory in many physiological processes. But when high levels of nitric oxide can cause damage to DNA, RNA,proteins and lipids.it leads to mutations and alters cell physiology that are hallmarks of the process of carcinogenesis.

In mitochondrial diseases,Phenotypic manifestation of the genetic diffect occurs only at when the threshold level is exceeded. This is named as ‘phenotypic threshold effect'

5) Describe several different principle mechanisms by which xenobiotics might affect our hormonal status. For each separate mechanism, give at least one example of a hormone whose level might be affected in this manner. What (which) basic property (properties) of the proteins involved make these mechanisms possible? (ANSWER = 1 A4 page, 10 points)

ANS: xenobiotics may affect various hormone levels in the cell.some chemical campounds have the ability to decrease female fertility.when diethylstilbesterol exposed to uterus rarely can leads to cervical and vaginal abnormalities in womans.another one is, oral contraceptives also can rarely produce persistent infertility in womans. The reason is that by hypothalamic pituitary and gonadotropin release. In addition,some of the environmental xenobiotics may have the ability to disrupt normal endochrine function by interfering with the binding of physiological ligands to steroid receptors and binding protein.


Effect on hormone


Mimics estrogen levels


Mimics estrogen and block testisteron


Inhibits testosteron


Alters thyroid hormone,decreases estrogen and testosterone level

Xenobiotic mediated P-450 expression: the enhanced expression of Cytochrom P-450(CYP) 2E1,CYP2B,CYP3A and CYP4A is leads to cause uncontrolled diabetes.because of the simaltanious and confounding metabolic and hormonal changes that occur in vivo.

Drug can lead to an increased expression of specific cytochrome P-450 proteins and as well as phase II drug metabolizing enzymes .NHR signal transduction pathways is considered mechanisms for the mediation of P-450 induction by xenobiotics. Its because of the receptors are activated by lipophilic ligands with molecular properties similar to those P-450 inducers.

6) 3-Methylcholanthrene causes liver cancer in rodents and pretreatment of these same animals with phenobarbital reduces the carcinogenic potency of a given dose of 3-methylcholanthrene. What is this kind of interaction called? Describe 5 different principle mechanisms by which phenobarbital might exert such an effect on the carcinogenicity of 3-methylcholanthrene. (ANSWER = 1 A4 page, 10 points)

7) Explain how a chemical compound such as carbon could have different toxic effects when engineered into carbon nanotubes than if it's in the form of charcoal?Also discuss what the responses of the immune system are towards carbon nanotubes? What could the adverse effects be due to these responses? (ANSWER = 1 A4 page, 10 points)

ANS: carbon nanotubes (CNT'S) are among most promising and innovative tools in the nanotechnology.which are very light and potentially can reach to lungs. The most research on this nano particles concluded that which are more toxic than carbon block,charcoal and quartze. It is considered serious occupational health hazard.CNT'S have the excellent mechanical,electrical and magnetic properties.

The research was conducted to identify toxicity of carbon nanotubes.five mices were died when treated with inhaling carbon nanotubes with in 7 days.so this issue is now concerned to environment as well as health of living systems.

The single-walled carbon nano tubes(SWCN's) could promote the proliferation of spleen cells. And the multi.walled carbon nanotubes(MWCN's) have the ability to suppress systemic immune function.in this mechanism involved, activation of cyclooxygenase enzymes in the spleen in response to a signal from the lung.

In recently researchers found that carbon nanotubes in the form of inhalation could effected the function of T cells.

Lovalace Respiratory Research Institute in Albuquerque. New Mexico, stated that “the nanotubes have appear to have interesting, subtle yet significant response, systemically, on different organ system that warrants careful consideration”

Inhaled carbon nano tubes may cause adverse effects on lung and other organ systems such as respiratory, pulmonary and systemic effects.

Carbon nano tubes possibility adverse effects to lung diseases and pre-existing respiratory diseases such as asthma etc.

8) Does oxidative stress contribute to life-span limitation in organisms? Give Pros and Cons. (ANSWER = 1 A4 page, 10 points)

ANS: the imbalance between the production of reactive oxygen and biological systems to readily detoxify the reactive intermediates. This is called as oxidative stress.

Free radicals,oxidants, and oxidative stress contribute to the aging process is a fundamental.but it is still controversial research topic.

In recent, schulz and their colleagues are experimentally shown that well controlled observations of 2-Deoxy-D-glucose ( DOG or 2-DG)- induced caloric restriction on life span in C.elegans. they were clearly observed that treatment with 2-DG increases life spane of C.elegans.and also increased mitochondrial respiration. They also found increase of reactive oxygen species (ROS). In contrast, animals could not show this kind of results. Effects of 2-DG on life span extension was abolished when treated with anti oxidants such as N-Acetylcysteine(NAC).

They are concluded that the glucose restriction induces mild oxidative stress. It is may originating from mitochondria and as well as increase protective mechanism.

Although the theory of oxidative stress is little acceptable, the mechanism involved in mammalians,aging poorly understood.

Redox homeostasis is highly dependant on both enzymatic and non-enzymatic pathways. the activity of pentose phosphate pathway increased when the cells exposed to oxidative stress conditions. this evidence shows reduction of NADP+ to NADPH.

For example, No life span effect was observance in mices, if even under the condition of overexpression SOD1 enzyme. Although increased thioredoxin was reported to increase life span in short lived mice.

One possible explanation is the lack of response to antioxidant over expression in some models is that whereas considerable oxidative damage builds up in the cell.

8) What can you in theory do yourself in attempt to avoid, eliminate or reduce oxidative stress and the damage it can cause the cells in your body? Describe in detail 5 principle approaches in this regards, including both how exogenous substances/stimuli can be used or avoided and how endogenous compounds and defense mechanisms can be influenced. (ANSWER = 1 A4 page, 10 points)

ANS: free radicals(reactive oxygen species) are rapidly reacts with other chemicals. it may leads to damage proteins,DNA and lipids.the presense of too many free radicals creats a condition called “oxidative stress” in the cell. oxidative stress hypothesis of aging, the hypothesis explains the level of oxidative stress will increase with age. This hypothesis also indicates increase level of antioxidants which reduces oxidative stress and extend the life span.

To avoid or eliminate or reduce oxidative stress, mainly antioxidants (ascorbic acid,glutathione,lipoic acid,carotenes and tocopherol etc.) and enzymes(catalase,SOD etc)are involved in this process.

General Reactions of antioxidants with free radicals:

R· + AH ------------>RH + A·

RO· + AH---------> ROH + A·

ROO· + AH-------à ROOH + A·

R· + A· ---------> RA

RO· + A· --------->ROA

ROO· + A· ---------> ROOA

Antioxidant + O2 ----------à Oxidized Antioxidant

Super oxide dismutases are enzymes catalysis the reaction to breakdown of super oxide anion into oxygen and hydrogen peroxide.SOD enzyme mostly present in all aerobic cells. Catalases are catalyze the reaction to conversion of hydrogen peroxide in to water and oxygen.

10) What are the principle factors that contribute to interindividual differences in medical drug metabolism (4points)? Discuss both practical and theoretical consequences of genetic polymorphism in the cytochrome P-450 system (6 point)? (ANSWER = 1 A4 page, total of 10 points)

ANS: the importance of genetic and environmental factors for interindividual difference in drug metabolism is difficult to identify. The rate of oxidative drug metabolism is under polygenic control.

Genetic factors: pharmacogenetics deals with variants definable