Biotransformation of drugs -part 4 | cytochrome P450 isozymes | CYP450 i...

Isozymes

CYP450 Enzymes are divided into
subgroups, Depending upon the sequence of amino acid in each isozyme.  These enzymes have the capacity to modify a large
number of structurally diverse substrates. In human beings, isozymes that fall
into families CYP1, CYP2, and CYP3 are primarily involved in the metabolism of
most drugs, in which the most active CYPs for drug metabolisms are those in the
CYP2C, CYP2D, and CYP3A subfamilies. In addition, an individual drug may be a
substrate for more than one isozyme. such as tolbutamide, paracetamol,
barbiturates, and nifedipine are substrates for more than one isoform. Which
will be discussed later in this session.

IMPORTANT ISOZYMES include

CYP3A4 which is thought to be the
most predominant CYP isoform involved in human drug metabolism, both in terms
of the amount of enzyme in the liver and the variety of drugs that are
substrates for this enzyme isoform. In addition to liver, these isoforms are expressed
in intestine, responsible for first pass metabolism at this site, and kidney as
well. This isoform may account for more than 50% of all CYP-mediated drug
oxidation reactions, and CYP3A4 is likely to be involved in the greatest number
of drug–drug interactions. However, the fact that two drugs are metabolized
predominantly by CYP3A4 does not mean that coadministration will result in a
drug–drug interaction, since drugs can bind in different regions of the CYP3A4
active site, and these regions may be different. In fact, it is believed that two
drugs can occupy the active site simultaneously, with both available for
metabolism by the enzyme. This finding helps account for several absent interactions
that would have been predicted to occur based on strict substrate specificity
rules.

CYP3A5, whose amino acid sequence is
like that of CYP3A4, appears to have approximately the same substrate
specificity characteristics as CYP3A4. However, it differs in that it is not present
in all individuals. Thus, individual expressing both CYP3A4 and CYP3A5 have the
potential to show increased metabolism of CYP3A drug substrates as compared to
individuals expressing only the CYP3A4 isoform. Drugs metabolized by these
isozymes include cyclosporine, simvastatin, HIV protease inhibitors, Ca channel
blockers, hydrocortisone, carbamazepine, midazolam, Losartan, and nifedipine.

The other identified human CYP3A
isoform is CYP3A7, which appears to be expressed in fetus and rapidly
disappears following birth, to be replaced by CYP3A4 and CYP3A5.

CYP2D6 This is the next most
important CYP isoform which metabolizes 25% of the CYP-mediated oxidation
reactions of drugs including tricyclic antidepressants, selective serotonin
reuptake inhibitors, antipsychotic agents, antiarrhythmics, β-blockers and opioid analgesics.

CYP2C8/9 this isoform handles about 16% of the CYP-mediated drug
oxidation reactions. This isoform metabolizes several clinically important
drugs with narrow therapeutic margins, two of these drugs is phenytoin and
warfarin, and other drugs like ibuprofen, tolbutamide, repaglinide, celecoxib
and losartan. CYP2C9 appears to prefer weakly acidic drugs as substrates, which
limits the number of drugs metabolized by this isoform since most drugs are
weak bases.

CYP2C19 isoform Metabolizes > 12
frequently used drugs including omeprazole, lansoprazole, naproxen, diazepam,
and propranolol.

CYP1A1/2 subfamilies take part in
the metabolism of only few drugs like theophylline, caffeine, tacrine,
phenacetin, paracetamol, and carbamazepine.

CYP2E1 isoform catalyzes metabolism
of only few drugs. like ethanol, halothane, enflurane, and paracetamol.

Large number of drugs are oxidized
through these pathways. Few drugs like cimetidine, ranitidine, clozapine are oxidized
by a group of flavin-monooxygenases that are also located at hepatic
endoplasmic reticulum, but are distinct from CYP enzymes. Some other drugs,
e.g. adrenaline, alcohol, mercaptopurine are oxidized by mitochondrial or
cytoplasmic enzymes.

When two coadministered drugs are
both metabolized by a single CYP, they compete for binding to the enzyme’s
active site. This can result in the inhibition of metabolism of one or both drugs,
leading to elevated plasma levels. If there is a narrow therapeutic index for
the drugs, the elevated serum levels may elicit unwanted toxicities. e.g., a
statin and a macrolide antibiotic or antifungal

Cytochrome P450 enzyme system - Pharmacology

Cytochrome P450 enzyme system drug metabolism Pharmacology:

Definition and general overview

Cytochrome P450 enzyme system Also known as microsomal mixed-function oxidases (MFOs) and cyp450 monooxygenases and simply abbreviated as CYP, p450 or CYP450 enzymes system.

The CYPs are a superfamily of isozymes made of haem proteins that catalyzes most of the phase 1 oxidation-reduction processes of drugs metabolic reactions.

The name cytochrome P450 is derived from the spectral properties of this hemoprotein; In its reduced mean ferrous form, it bind with carbon monoxide to form a pink compound, which shows maximum absorption at 450 nm, that’s why they are named as p450,

In humans, over 50 individual P-450s have been identified but only about 12 are involved in the metabolism of most drugs. each member of which catalyzes the biotransformation of a unique spectrum of drugs, with some overlap in the substrate specificities and may act on the same substrates but at different rates. The only common feature of the many drugs metabolized by this pathway is lipid solubility.

The CYPs carry out drug metabolism and metabolize many structurally diverse chemicals. This is due to the multiple forms of CYPs and the capacity of a single CYP to metabolize many structurally distinct drugs. In addition, CYPs can metabolize a single compound at different positions on the molecule. the CYPs are considered unselective to bind and metabolize multiple substrates.

These enzymes are also responsible for all or part of the anabolism and catabolism of a number of endogenous compounds, such as steroid hormones, bile acids and prostaglandins.

Structure

Heme protein of the cytochrome p450 contains one atom of iron in a hydrocarbon cage that functions to bind oxygen during the reaction. Many other enzymes that use O2 as a substrate for their reactions contain heme.  E.g. hemoglobin. These enzymes catalyze an oxidation-reduction processes that requires CYP450, CYP450 reductase, NADPH (reducing agent), and O2.

 location

Many drug-metabolizing enzymes are located in the lipophilic endoplasmic reticulum membranes of the hepatic cells which has the greatest specific enzymatic activity, and other sites like GIT and Kidneys.

Nomenclature

CYPs are named with the root CYP followed by a number appointing the family, a letter denoting the subfamily, and another number naming the CYP form. Thus, CYP3A4 is family 3, subfamily A, and gene number 4.

Mechanism of drugs metabolism

These enzymes catalyze reactions that requires CYP450, CYP450 reductase, NADPH, and oxygen. First of all, in step 1, P450 containing ferric iron (Fe3+) combines with a molecule of drug (RH). And form a complex. Subsequently, in second step, NADPH donates an electron to the flavoprotein P450 reductase, and the flavoprotein is reduced from oxidized form which in turn reduces the iron to ferrous form (Fe2+), In third step, it combines with molecular oxygen and subsequently, combines with a proton and a second electron from flavoprotein P450 reductase to form an activated oxygen-P450-substrate (Fe2+OOH–RH) complex. This combines with another proton to yield water with the liberation of oxidized drug from the complex in the next step and regeneration of P450 enzyme.

In this oxidation-reduction process, two microsomal enzymes play a key role which are NADPH cytochrome P450 oxidoreductase and cytochrome P450.

In the overall reaction, the drug is oxidized, and oxygen is reduced to water. The mechanism involves a complex cycle but the overall net effect of the reaction is quite simple, the addition of one atom of oxygen to the drug to form a hydroxyl group, the other atom of oxygen being converted to water.

Cytochrome P-450 catalyzes several reactions, including aromatic and aliphatic hydroxylation reactions, dealkylation at nitrogen, sulfur, and oxygen atoms; heteroatom oxidations at nitrogen and sulfur atoms; reductions at nitrogen atoms; and ester and amide hydrolysis