Lindsay Pasquale
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Phase I and II Biotransformation
Phase I of metabolism is the introduction of a pair of modification
or functional group in a drug molecule such that it becomes more polar. There
are different types of Phase I metabolism reactions; oxidation, reduction, and
hydrolysis. First, oxidation is the introduction of OH enzymes; mixed function
oxidase, monooxygenases, cytochrome, association with reductase enzyme, in
other words contains NADPH. Examples of oxidative enzymes are Flavin containing
monooxygenases; nucleophilic atoms. Secondly, alcohol dehydrogenases; alcohols
to aldehydes and ketones. Lastly, aldehyde dehydrogenases; which are aldehydes
to carboxylic acids. Reduction interacts with the reducing agents, Azo- and
Nitro-reductions can be catalyzed. This process happened by the enzymes of
intestinal flora. And also by cytochrome P450, usually known as the oxidizing enzyme,
has the capacity to reduce xenobiotic under low anaerobic or oxygen conditions.
Reduction participates in the role of intestinal microbial flora in
biotransformation. Hydrolysis is a chemical reaction of a compound with water,
usually resulting in the formation of one or more compounds. Epoxide Hydrolase
(EH) is detoxifying enzyme for epoxides, it’s the formation of diols. Epoxide
can be present in many tissues; epoxides are electrophilic which tend to bind
to proteins or to nucleic acids. The role of EH in biotransformation of
benzoapyrene is the inactivation, and the conversion of benzoapyrene to
tumorigenic diol epoxide.
The chemical role of Phase II reactions in biotransformation is the
process of an organic acid, acetyl, or methyl group is conjugated to the
molecule at a preexisting functional group or at a functional group acquired in
phase I biotransformation. Phase II includes; glucuronidation, sulfation,
conjugation with glycine, conjugation with glutathione, acetylation, and
methylation. Glucuronide conjugation is the acid from glucose, that reduces
toxicity and sometimes produce carcinogenic substances. Excretion in the kidney
or bile depending on conjugate size. This process includes xenobiotics and also
endogenous substances. Sulfate conjugation has the ability to decrease toxicity,
it is readily excreted by urine. The most common sulfo group is sulfotransferase,
the transferring of enzymes that catalyze the transfer of a sulfo group from a
donor molecule to an acceptor alcohol or amine. Lastly in the sulphate
conjugation, PAPS limits the pathway. Acetylation is the water solubility of
parent molecule and their excretion. It masks the functional group of parent
from getting involved in conjugations. Methylation reaction makes slightly less
soluble. It masks available functional groups, different types include;
O-methylation, N-methylation, and S-methylation.
Phase I is the parent drug that is altered by introducing or
exposing a functional group. Drugs that are transformed by the reaction of this
phase usually loose pharmacological activity. Phase I reactions convert
inactive, prodrugs to biologically active metabolites. Reaction products have
the possibility to be directly excreted in the urine, or react with endogenous
compounds o form water soluble conjugates. Phase II is a parent drug that
participates in conjugation reactions that form covalent linkage between a
parent compound functional group and glucuonic acid, sulfate, glutathione,
amino acid, an acetate. The organ for biotransformation is the liver, but other
organs participate in metabolism.
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