the smooth endoplasmic reticulum is concerned with biotransformation and contains
enzyme components while the rough endoplasmic reticulum is mainly concerned
with protein synthesis (Buhler and Williams 1988).
These are mainly present in tissues like liver, kidney, lungs, and intestinal
mucosa. These can be induced by drugs and/or diet. This category contains—
Cytochrome P450 (CYPs), Flavin-containing monooxygenase (FMOs), UDPglucuronosyltransferase (UGTs), etc. (Fig. 16.6) Enzymes of this category are
non-soluble proteins. The microsome containing Cytochrome P450 (ferric, ferrous
forms), NADPH (flavoprotein), membrane lipids, molecular oxygen is termed as
microsomal mixed function oxidase system (Parkinson 2001; Timbrell and Marrs
2009).
The microsomes are very useful for researchers to mimic the properties of the
endoplasmic reticulum in a test tube. They use hepatic microsomal preparations from
various species for determination of phase I biotransformations of xenobiotics
(drugs). The CYP enzyme family present in microsomes is responsible for phase I
biotransformations and these tests are the primary source of information on the
metabolism of any test compound. For this determination, the test compound is
typically incubated in approximately 1.0 mg/ml microsomal protein, phosphate
buffer, pH 7.4 at 37
C for 30 min (Vrbanac and Slauter 2013). Its stability is then
checked by an appropriate analytical method such as LC-UV or LC-MS (Vrbanac
and Slauter 2013). Viable drug candidates should have a species-specific, predetermined percentage remaining under these conditions.
16.5 Non-enzymatic Elimination
It is a process of spontaneous, non-catalyzed type of biotransformation which takes
place at physiological pH for highly active, unstable compounds (Smitha et al.
2017).
The substrate can react with any of the following to result in a non-enzymatic
reaction:
1. Reactions with endogenous nucleophiles: Endogenous water, thiols, cysteine act
as endogenous nucleophiles. Example: Oxidation of nitrosobenzene in red blood
cells.
2. Reactions with endogenous electrophiles: Biogenic aldehydes and ketones form
endogenous electrophiles. Example: Formation of hydrazones from hydralazines.
3. Breakdown and rearrangement in acidic and neutral aqueous media. Example:
non-enzymatic conversion reaction of 6-chloromethyl and 6-bromomethyl to 6hydroxymethyl BP at physiological conditions. 80–90% of the incubated amount
was transformed in 1 h.
4. Reactions between themselves (two drug or their metabolites): such reactions
usually involve adduct formation. Example co-administration of acetylsalicylic
acid and acetaminophen to model animals leads to the formation of salicylic acid
348
M. H. Yashavarddhan et al.
enzyme components while the rough endoplasmic reticulum is mainly concerned
with protein synthesis (Buhler and Williams 1988).
These are mainly present in tissues like liver, kidney, lungs, and intestinal
mucosa. These can be induced by drugs and/or diet. This category contains—
Cytochrome P450 (CYPs), Flavin-containing monooxygenase (FMOs), UDPglucuronosyltransferase (UGTs), etc. (Fig. 16.6) Enzymes of this category are
non-soluble proteins. The microsome containing Cytochrome P450 (ferric, ferrous
forms), NADPH (flavoprotein), membrane lipids, molecular oxygen is termed as
microsomal mixed function oxidase system (Parkinson 2001; Timbrell and Marrs
2009).
The microsomes are very useful for researchers to mimic the properties of the
endoplasmic reticulum in a test tube. They use hepatic microsomal preparations from
various species for determination of phase I biotransformations of xenobiotics
(drugs). The CYP enzyme family present in microsomes is responsible for phase I
biotransformations and these tests are the primary source of information on the
metabolism of any test compound. For this determination, the test compound is
typically incubated in approximately 1.0 mg/ml microsomal protein, phosphate
buffer, pH 7.4 at 37
C for 30 min (Vrbanac and Slauter 2013). Its stability is then
checked by an appropriate analytical method such as LC-UV or LC-MS (Vrbanac
and Slauter 2013). Viable drug candidates should have a species-specific, predetermined percentage remaining under these conditions.
16.5 Non-enzymatic Elimination
It is a process of spontaneous, non-catalyzed type of biotransformation which takes
place at physiological pH for highly active, unstable compounds (Smitha et al.
2017).
The substrate can react with any of the following to result in a non-enzymatic
reaction:
1. Reactions with endogenous nucleophiles: Endogenous water, thiols, cysteine act
as endogenous nucleophiles. Example: Oxidation of nitrosobenzene in red blood
cells.
2. Reactions with endogenous electrophiles: Biogenic aldehydes and ketones form
endogenous electrophiles. Example: Formation of hydrazones from hydralazines.
3. Breakdown and rearrangement in acidic and neutral aqueous media. Example:
non-enzymatic conversion reaction of 6-chloromethyl and 6-bromomethyl to 6hydroxymethyl BP at physiological conditions. 80–90% of the incubated amount
was transformed in 1 h.
4. Reactions between themselves (two drug or their metabolites): such reactions
usually involve adduct formation. Example co-administration of acetylsalicylic
acid and acetaminophen to model animals leads to the formation of salicylic acid
348
M. H. Yashavarddhan et al.