165 Bfcp2B Biochemistry Cytochrome P450

Front 1

Describe the Phase I stage of drug detoxfication.

Back 1

Cytochrome P450s are a major class of enzymes that funciton to transform lipophylic xenobiotics into more polar metabolites via the addition of a polar group.

Front 2

Describe the phase II stage of drug detoxification.

Back 2

Phase enzymes are primarily transferases that perform conjugation reactions such as glucuronidation, sulfation, and glutathione conjugation. These conjugates are more hydrophilic and are more easily excrete that parent compounds.

Front 3

What are the dangers of the Phase I and Phase II stage of drug detoxification?

Back 3

They can actually activate xenobiotics causing toxicity.

Front 4

What are cytochrome P450s?

Back 4

A ubiquitous superfamily of mixed function oxidases involved with Phase I detoxification that are defined by the present of a heme prosthetic group coordinated by a cysteine thiolate ion. They use molecular oxygen to hydroylate a substance.

Front 5

Where are P450s most common?

Back 5

They are found in all tissues but are enriched in liver, lung, and intestinal mucosa. They are integral membrane proteins that are primarily found in the ER other than CYPs.

Front 6

What are the important features of the cytochrome P450 catalytic cycle?

Back 6

The cycle involves the generation of a number of reactive oxygen species during the transfer of an electron that requires high fidelity of the enzyme to prevent oxidative stress.

Front 7

What is the role of cytochrome P450 reductase (CPR) in detoxification?

Back 7

It allows for the sequential transfer of electrons from NADPH or NADH to CYP for a hydroxyltion reaction.

Front 8

What is the most common Phase II reaction?

Back 8

Conjugation with glucoronic acid via uridine diphosphate-alpha-D-glucuronic acid (UDPGA). (Works with compounds with hydroxyl, amino, or sulfhydryl groups.)

Front 9

What protein serves a major role in dealing with reactive oxygen species in a Phase II reaction?

Back 9

Glutathione-S-transferase (GST) is present in high quantities (aobut 10% of soluble protein) in hepatocytes.

Front 10

What phase II protein transfers a sulfate to enhance solubility?

Back 10

Sulfotransferase (SULT)

Front 11

What phase II protein transfers an acetyl group and has a couple of isoforms that can be inhibited by polyphenolic compounds like caffeine or coumadin?

Back 11

N-acetyltransferase (NAT)

Front 12

Which phase II protein catalyzes S-methylation reactions that are important in processing chemotherapeutic drugs?

Back 12

Thiopurine S-methyltransferase (TPMT)

Front 13

What is an unintended consequence of some CYP Phase I or enzyme Phase II reactions?

Back 13

Activation of xenobiotics may lead to generation of reactive oxygen species, generation of highly toxic hydroxyl radicals, or generation of other reactive intermediates.

Front 14

What are some mechanisms of cytochrome P450 regulation?

Back 14

1. Environmental influences 2. Biological factors 3. Genetic polymorphisms

Front 15

What are some factors that lead to reduced expression of cytochrome P450 enzymes?

Back 15

1. Disease states 2. Aging 3. Competitive inhibition (e.g. CYP3A4 metabolizes many different drugs) 4. Mechanism-based inhibition by drugs 5. Genetic polymorphisms

Front 16

What are some important CYP genetic polymorphisms?

Back 16

1. CYP2C9 metabolize NSAIDs and may have complications with drugs with a narrow therapeutic index 2. CYP2C19 metabolize proton-pump inhibitors, lower metabolism can help with H. pylori 3. CYP2D6 lower rates of metabolism of antidepressants and analgesics (important for prodrugs tramadol & codein)

Front 17

What might be a cause of increased expression of P450 enzymes?

Back 17

There are a number of mechanisms that may lead increased expression of enzyme including de novo synthesis (via XRE promoters) and other post-transcriptional mechanisms.

Front 18

Which 6 most common CYPs metabolize over 90% of prescription drugs?

Back 18

1. CYP3A4 2. CYP2C9 3. CYP2D6 4. CYP2C19 5. CYP1A2 6. CYP2C8

Front 19

Which CYP isoform is most often involved in drug-drug interactions?

Back 19

CYP3A4

Front 20

What are 3 common dietary effectors of CYPs?

Back 20

1. Grapefruit juice inhibits CYP3A 2. Cruciferous vegetables induce CYP1A1 3. Iron deficiency limits heme synthesis which is needed for all CYPs

Front 21

What protein is essential in excreting drug conjugates across the hepatocyte canlicular and renal luminal membranes (important for final state of detox)?

Back 21

Multi-drug resistance protein (MRP) transporters

Front 22

What is the mechanism by which dioxin exhibits its poisonous effects?

Back 22

Dioxin upregulates and binds very tightly to CYPA1A. Due to the fact that it's half life is about 10 years, a lot of reactive intermediates are generated for a very long time during its processing.

Front 23

What is the mechanism for acetaminophen toxicity?

Back 23

Most acetaminophen is processed by conjugation with glutathione (about 5% is handled by CYP2E1 generating a higly reactive hepatoxic compound known as N-acetyl-p-benzoquinoneimine (NAPQI)). When the system is overwhelmed, a lot of this acetaminophen gets processed by CYP2E1 due to reduced glutathione stores leading to toxicity. Drinking also induces CYP2E1 and fasting reduces UDPG further complicating the problem.

Front 24

What's an antidote for acetaminophen toxicity?

Back 24

Administration of N-acetylcysteine (NAC)

Front 25

What paradoxical fact is true with respect to ethanol and a P450 enzyme?

Back 25

Alcohol is both a substrate and induces of CYP2E1. This is important in acetaminophen toxicity.

Front 26

What are two well known inducers of CYP3A?

Back 26

Rifampin & St. John's Wort