4 Biotransformation

Dr. Yogalakshmi K N and Dr. K N Sangeetha

Objectives:

1. To familiarize the concept of biotransformation
2. To understand the types and consequences of biotransformation
3. To study the factors influencing biotransformation

Biotransformation

Xenobiotics and their sources

Xenobiotic is defined as a chemical or molecule that is foreign to and exerts a variety of effects on the biological system. These effects may be beneficial, in the case of drugs, or deleterious, in the case of poisons. The origin of these toxins that are present in our bodies commonly referred to as xenobiotics comes from an array of sources. These sources include exogenous sources that include environmental exposure; such as in the air we breathe, the food we eat, the water we drink, and medications; and the endogenous sources such as; the products produced by digestion, energy metabolism, tissue regeneration, and end products from the metabolism of hormones, bacterial by-products and other complex molecules. Other xenobiotics includes natural and manufactured chemicals such as drugs, industrial chemicals, pesticides, pollutants, pyrolysis products in cooked food, alkaloids, secondary plant metabolites, and toxins produced by molds, plants, and animals.

Need for biotransformation

Xenobiotics are mostly lipohilic in nature due to which they are easily absorbed through the skin, lungs, or gastrointestinal tract and cannot be easily eliminated from the body. Detoxification is the process of transforming and removing xenobiotics that are potentially harmful products from the body. The detoxification process has its own energy, nutrient and regulatory requirements. Elimination of xenobiotics often depends on their conversion to water-soluble chemicals. So the process of transforming toxins into a form suitable for excretion is called biotransformation. Without biotransformation, lipophilic xenobiotics would be excreted from the body so slowly that they would eventually overwhelm and kill an organism

What is biotransformation?

Biotransformations are structural modifications of a chemical compound by organisms /enzyme systems that lead to the formation of molecules with relatively greater polarity. This chemical process of modification of endogenous or exogenous chemicals into water soluble chemicals aid in the easy elimination of the toxic/foreign chemicals from the body. Natural Biotransformation mechanisms are catalyzed by enzymes in the liver and other tissues. However, the natural transformation process are slow, nonspecific and less productive. There are also other types of biotransformation reactions catalysed by microbes called microbial biotrasformation which was developed to acclimatize to environmental changes making them useful in several biotechnological processes. The most significant aspect of biotransformation is that it maintains the original carbon skeleton after obtaining the products. Biotransformations can also be used to synthesize compounds or materials where synthetic approaches are challenging.

Types of biotransformation

Biotransformation is of two types:

• Enzymatic
• Non-enzymatic

Enzymatic biotransformation is the process of elimination of chemicals from the human body catalysed by various enzymes present in the body. In the body, enzymes metabolizing the drugs and xenobiotics are different from food metabolizing enzymes. They are broadly classified into

1. microsomal
2. non microsomal

Microsomal biotransformation is caused by enzymes present within the lipophilic membranes of smooth endoplasmic reticulum. Microsomal enzymes are located on smooth endoplasmic reticulum of liver, kidney, lungs, intestinal mucosa and catalyse majority of drug biotransformation reaction that includes oxidative, reductive, hydrolytic and glucouronidation reactions. The enzymes are inducible by drugs and diet.

Examples of microsomal enzymes include

• Cytochrome P450 (ferric, ferrous forms)
• (CYPs) NADPH (flavoprotein)
• Flavin monooxygenase systems (FMO)
• Epoxide hydrolase (EH)
• Uridine 5′-diphosphate-glucuronyltransferase (UDPGT)
• Molecular oxygen
• Membrane lipids

Non-Microsomal Biotransformation are catalysed by non-microsomal enzymes that are non-specific enzymes. These enzymes are located in the cytoplasm and mitochondria of liver cells, plasma and also in other tissues. They are not inducible but show polymorphism. They catalyse very few oxidative; a number of reductive; and hydrolytic reactions. They also show conjugation reaction except for glucouronidation.

Examples:

• Xanthine oxidase converting hypoxanthine into xanthine. Cytotoxic agent 6-mercaptocurine
• Monoamine oxidase involved in non-microsomal metabolism of catecholamines and noradrenaline.
• Alcohol dehydrogenase responsible for metabolism of ethanol into acetaldehyde
• Tyrosine hydrolases enzymes

The metabolizing enzymes that catalyse biotransformation are either less or completely absent in the neonates leading to decreased metabolism of drugs. This makes the drugs toxic. For example certain syrups are toxic for children despite of minimum alcohol percentage. Drug metabolizing enzymes are controlled by specific genes coding for specific protein (enzymes). However, the specific gene coded proteins vary from species to species. For example, dogs are deficient in acetyl transferase whereas cats are deficient in glucouronyl transferase. Also, some drugs are inactivated in the body fluids by

spontaneous molecular rearrangement without the presence of any enzyme. They are eliminated from the body due to hepatic dysfunction. Example: atracurium

Non enzymatic Biotransformation

Spontaneous, non-catalyzed and non-enzymatic types of biotransformation are for highly active. It produces unstable compounds at varying physiological pH. Some of the example include

• Mustin HCl converted into Ethyleneimonium
• Atracurium converted into Laudanosine and Quartenary acid
• Hexamine converted into Formaldehyde
• Chlorazepate converted into Desmethyl diazepam

Role of biotransformation

Biotransformation causes

• The polarity of the drugs increases after metabolism in the body. This favours easy excretion of the drugs
• Alteration or termination of drug activity
• Converts pro-drug to active compound
• Coverts less active drug to more active drug
• Produces toxic compound

Consequences of biotransformation

1. The physical properties of xenobiotics are generally changed from those favouring absorption (lipophilicity) to those favouring excretion in urine or fecaes (hydrophilicity). However, an exception is the elimination of volatile compounds by exhalation, wherein biotransformation to non-volatile, water-soluble chemicals can retard their rate of elimination. Similarly, biotransformation of xenobiotics in the brain and testis (two organs with a barrier to chemical transport) might also be an obstacle to xenobiotic elimination if the metabolites cannot cross the blood-brain or blood-testis barrier.
2. Xenobiotics exert a variety of effects on biological systems that may be beneficial or deleterious depending on the physicochemical properties of the xenobiotic. Example, in pharmacology, chemical modification of drugs by biotransformation helps the drug to exert its pharmacodynamic effect (i.e., it is the metabolite of the drug, and not the drug itself, that exerts the pharmacologic effect).

3. In toxicology, many xenobiotics must undergo biotransformation to exert their characteristic toxic or tumorigenic effect (i.e., many chemicals would be considerably less toxic or tumorigenic if they were not converted to reactive metabolites by xenobiotic-biotransforming enzymes). However, biotransformation terminates the pharmacologic effects of a drug and lessens the toxicity of xenobiotics. Enzymes catalyzing biotransformation reactions often determine the intensity and duration of action of drugs and play a key role in chemical toxicity and chemical tumorigenesis.

Biotransformation capacity of the organism

Biotransformation capacity of the organism depends on the degree to which the organisms are exposed to xenobiotics. For example, insects that feed on a variety of plants have a greater capacity to biotransform xenobiotics than insects that feed on a limited number of plants. Compared with mammals, fish have a low capacity to metabolize xenobiotics because they can eliminate xenobiotics unchanged across their gills.

Other factors affecting biotransformation

Presystemic Metabolism/First pass effect/Route of Administration

First pass effect or presystemic metabolism is a phenomenon of drug metabolism wherein the concentration of drug is greatly reduced before it reaches the systemic circulation. Drugs following first pass metabolism have decreased bioavailability. Most of the drugs are metabolized within the liver. Changing the route of administration might change the first pass metabolism. For example Propanolol is 80% metabolized before reaching systemic circulation.

Genetic Variations

Drugs behave differently in different individuals due to genetic variations. These inter individual genetic variations results either in complete absence of the enzyme or its malformation leading to malformed genes. Mostly non microsomal enzyme show genetic variations. Examples: Succinyl choline, a skeletal muscle relaxant is metabolized by pseudocholine esterase. Some people lack this enzyme, due to which metabolism of succinyl choline might not occur. When administered in those individuals, prolonged apnea might result. Different groups of populations might be classified as fast metabolizers and poor metabolizers of drugs. For certain drugs, like isoniazid, fast acetylators as well as slow acetylators are present. Fast acetylators cause rapid acetylation, while poor metabolizers metabolize less. Hepatic acetyl transferrase catalyzes acetylation. Slow acetylation might occur due to genetic malformation leading to decreased production. Fast metabolism may lead to hepatotoxicity while poor metabolism might result in peripheral neuropathy. Absence of catalase may lead to achalasia, while G6PD deficiency predisposes erythrocytes to hemolysis as a result of oxidative stress imposed by some commonly used drugs.

Species Differences

Drug metabolizing capacity varies between species. The difference is most established in animals. Some metabolize drugs rapidly and some do not. Example: Rats and rabbits metabolize drugs more efficiently than humans. Likewise, in human drug metabolizing capacity varies between races. Example: Drug metabolizing capacity of certain anti-malarial drug is different amongst the Eskimos and the Asians. Eskimos may metabolize drugs more efficiently than the Asians.

Exposure to Pollutants from Environment or Industry

Exposure to external factors such as pollutants and chemicals affects the enzymes of the biotransformation reaction resulting in enhanced or inhibited biotransformation. Smoking and chronic alcoholism might act as enzyme inducers. Similarly, pesticides or insecticides may act as enzyme inducers. Likewise, environmental factors too influence and show effect on biotransformation. In hot and humid climate biotransformation is decreased and vice versa. At high altitude, the occurrence of biotransformation decreases due to decreased oxygen leading to decreased oxidation of drugs.

Age

Age plays a very important role in drug metabolism by biotransformation. Extreme age groups (very young and very old) behave almost the same. Drug metabolizing enzyme develop early but their capacity is low. Thus the rate of metabolism in infants is very low. Care should be taken in administering drugs in younger patients. True development of enzyme occurs in one to two months. Chloramphenicol (antimicrobial drug) when administered in infant, does not have great efficacy. Toxic effects in the form of grey baby syndrome might occur. The baby may be cyanosed, hypothermic, flaccid and grey in color. Shock and even death might occur if toxic levels get accumulated. Diazepam (sedative hypnotic) may result in floppy baby syndrome in which flaccidity of the baby is seen.

In elderly people, most processes slow down and leads to decreased metabolism. Shrinkage of organs occurs along with decreased liver functions and decreased blood flow through the liver. All these factors decrease the metabolism. Old people that are usually taking multiple drugs are more prone. Henceforth, the drug doses should be decreased in the elderly persons.

Sex

Male have a higher BMR as compared to the females, thus can metabolize drugs more efficiently, e.g. salicylates (others might include ethanol, propanolol, benzodiazepines). Females, during pregnancy, have an increased rate of metabolism. Thus, the drug dose has to be increased. After the pregnancy previous doses can be high and hence the dosage should be decreased and reverted back to normal levels. Example includes phenytoin, whose dose has to be increased during pregnancy (specially second and third trimester).

Drug-Drug Interaction

Toxic effects might result when drug combinations act as enzyme inhibitors and inducers. The dose has to be adjusted accordingly.

Nutrition

Malnutrition may also affect biotransformation. Depletion of amino acids and glycine may affect drug metabolizing capacity, especially during the phase II, which depends on the food stores. Synthesis of microsomal enzymes depend on nutritional status.

Pathological Conditions

Most of the drugs are metabolized in the liver. Any disease which might cause cirrhosis, viral hepatitis, drugs induced hepatitis, hepatocarcinoma may slow down the metabolizing capacity. Jaundice depresses glucuronic acid conjugation and oxidative function of liver microsomes. Likewise, cardiovascular diseases, although have no direct effect, decrease the blood flow, which may slow down biotransformation of drugs like isoniazid, morphine and propanolol. Similarly pulmonary conditions may decrease biotransformation. Procaine and procainamide hydrolysis is also impaired. Hypothyroidism increases drug metabolizing capacity (increased half-life of antipyrine, digoxin, methimazole, practolol) while hyperthyroidism decreases it.

Circadian rhythm

The rate of hepatic metabolism of certain drugs follow diurnal rhythm in rats and mice. This may be true in humans as well.

Advantages of Biotransformation

1. Significance of bioconversion reactions becomes obvious when the production of a particular compound is difficult by chemical methods
2. Biotransformation reactions are highly preferred as they are cheaper, more direct than their chemical analogues and the conversion normally proceeds under conditions that are regarded as environmentally acceptable.
3. They are generally preferred to chemical reactions because of substrate specificity, stereo specificity and mixed reaction conditions (pH, temperature, and pressure).
4. In biotransformation, the enzymes or whole cells provide a remarkable enhancement in reaction rates as well as specificity to exhibit high stereoselectivity over the corresponding reactions.
5. Microbial transformation offers the advantages of highly selective operation at non extreme pH, near room temperature and reduced levels of toxic waste products.
6. Biotransformation with recombinant microbial enzymes have been widely used, including applications for the production of hormones, antibiotics, and speciality chemicals.
7. The environmental pollution due to biotransformation is almost insignificant or negligible.
8. In addition, it is easy to apply recombinant DNA technology to make desired improvements in bio-transformations.
9. Another practical advantage of bio-transformations is that it is easy to scale-up the processes due to limited number of reactions.

Summary

At the end of this module we have studied about

• Concept of biotransformation
• Need of biotransformation
• Advantages and disadvantages of biotransformation
• Types and consequences of biotransformation
• Factors influencing biotransformation

References

1. Kieslich K (1984) Biotechnology. A Comprehensive treatise in 8 volumes. Rehm H-J and Reed G. (eds.) Biotransformation 6: 1.
2. Lilly MD (1984) Advances in biotransformation processes. Trans Inst Chem Eng 72: 27-34
3. Andrew Parkinson, Biotransformaton of xenobiotics