Phase One Monooxygenases: Example Reactions

Epoxidation and Hydroxylation Reactions, cont.:

• As in organic chemistry can get ortho- and meta- directing substituents:

Unsaturated Aliphatic Hydroxylations:

Aliphatic Hydroxylations:

Aliphatic compounds are not readily oxidized or metabolized unless there is an aromatic side chain. For aromatic substituted compounds initially get 1° & 2° alcohols:

Alicyclic Hydroxylations:

Alicyclic compounds are preferentially hydroxylated on the aliphatic parts (aromatic hydroxylation occurs as a minor product and is not shown):

Heterocyclic Hydroxylations:

Heterocyclic compounds are hydroxylated on the three position:

Dealkylation Reactions:

N-dealkylations: Alkyl groups are hydroxylated adjacent to nitrogen and rearrange to release an aldehyde as shown below for the dealkylation of N,N-dimethyl-p-nitrophenylcarbamate:

and the didealkylation of N,N-dimethylaniline:

O-dealkylations:

Alkyl groups are hydroxylated adjacent to oxygen and rearrange to release an aldehyde as shown below for the dealkylation of p-Nitroanisole:

and the dealkylation of Phenacetin:

With organo-phospho triesters the dealkylation occurs with the ester instead of the ether as shown for Chlorfenvinphos:

S-Dealkylation:

N-Oxidation

Hydroxylamine formation:

In this instance the product is thought to be responsible for the formation of methemoglobin following aniline administration.

In another reaction 2-acetylaminofluorine is activated to a potent carcinogen, N-hydroxy-2-acetylaminofluorine:

Oxime formation:

Deamination:

S-Oxidation

For sulfur compounds such as sulfides, thiols, thioethers (shown), etc. get a common pattern of oxidation to the sulfoxide and then further oxidation to the sulfone:

Desulfuration

The replacement of sulfur by oxygen results in the activation of a number of insecticides, such as Parathion, below, where the Paraoxon product is the more active substance:

P₄₅₀ Mechanism

Now that we've seen these reactions, let's go back and look at the P₄₅₀ Mechanism. First, recall the microsomal electron transport system:

Next note the mechanism from your text [overhead from text, p 79] and the heme structure.

Note that only four of the six ligand bonds to iron are shown. The fifth ligand bond (perpendicular to the page) goes to a cysteine sulfur on the protein with the sixth bond (opposite to the fifth) available to ligand oxygen, water, etc.

Let's now look at the P₄₅₀ Mechanism is some detail-overhead from text, heme structure. (Read Timbrell, pp 77-82 on the Mono-oxygenase system.)

Non-P₄₅₀ Based Oxidations

Microsomal oxidations: (Largely sER localized enzymes)

Amine oxidase:

Non-microsomal oxidations: (Enzymes of the mitochondrial or soluble fraction of cytosol)

Amine oxidation:

Monoamine oxidases (MAO) - flavoproteins in the mitochondria. Primary amines are preferred:

Diamine oxidases (DAO) - diamines to charectristic aldehydes

DAO oxidizes primary diamines only. The rate depends on the chain length, with a maximum at four carbons (putrecine) and five carbons (cadaverine), and going to 0 at nine or above.

At nine carbons and above MAO becomes active, since the separation of the amine groups is sufficient for the substrate to look like a monoamine.

Alcohol and Aldehyde oxidation:

Regardless of ethanol consumption its metabolism is important since it is endogenously produced in the large intestine by the microbial fauna to give about 0.5 mM ethanol in the hepatic portal vein! (Compare to the concentration reached after a single drink of 2-5 mM.)

• Most ethanol metabolism takes place in the liver, with the primary pathway catalyzed by Alcohol Dehydrogenase (ADH) at low concentrations of ethanol since the K_M of ADH is low (approximately 1mM):

• Ethanol is also oxidized by the microsomal ethanol oxidation system (MEOS), which is P₄₅₀ based:

This reaction may explain ethanol's competition with drug metabolism.

• Ethanol can also be oxidized by Catalase:

The aldehyde produced in alcohol metabolism can be oxidized further:

Aldehyde oxidation is mostly due to mitochondrial aldehyde dehydrogenase in liver (K_M = 0.01mM, cytosolic = 1 mM). The mitochodrial enzyme also has a greater capacity.

Some of the toxic effects of ethanol on the liver are due to NAD⁺/NADH.

• Ethanol can lower liver NAD⁺/NADH from 1000-100! Since most dehydrogenases operate at equilibrium, this can affect metabolism generally! (e.g. Glycolysis, gluconeogenesis).
  • Decreased NAD⁺/NADH markedly decreases pyruvate concentrations.
  • Ethanol is a potent inhibitor of gluconeogenesis, which could contribute to hypoglycemia in alcoholics and to lactic acidosis.
• Ethanol also decreases testosterone concentrations.

While other effects are related to acetaldehyde production.

• Acetaldehyde is responsible for many effects of ethanol even though its concentration in the blood is only 0.02-0.23mM.
  • It can get higher concentrations if aldehyde dehydrogenase is inhibited by pyrogallol and other alliphatic aldehydes present in beverages.
  • Acetaldehyde inhibits mitochondrial oxidative phosphorylation, decreasing ATP/ADP.
  • Acetaldehyde reacts with biogenic amines to neurotransmitters, possibly accounting for some of alcohol's neurologic effects etc.

Last modified 2 March 2010

© RA Paselk 2001