Meta-Biol

• Pathways

The P450 isozyme system is the major phase 1 biotransforming system in man, accounting for more than 90% of drug biotransformations. This system has huge catalytic versatility and a broad substrate specificity, acting upon xenobiotica and endogenous compounds. It is also called the mixed-function oxidase system, the P450 monooxygenases and the heme-thiolate protein system. All P450 enzymes are a group of heme-containing isozymes which are located on the membrane of the smooth endoplasmic reticulum. They can be found in all tissues of the human body but are most concentrated in the liver. The name "cytochrome P450" (CYP) is derived from the spectral absorbance maximum at 450nm when carbon monoxide binds to CYP in its reduced (ferrous, Fe²⁺) state. The basic reaction catalyzed by CYP is mono-oxygenation, that is the transfer of one oxygen atom from molecular oxygen to a substrate. The other oxygen atom is reduced to water during the reaction with the equivalents coming from the cofactor NADPH. The basic reaction is;

The end results of this reaction can be (N-)hydroxylation, epoxidation, heteroatom (N-, S-) oxygenation, heteroatom (N-, S-, O-) dealkylation, ester cleavage, isomerization, dehydrogenation, replacement by oxygen or even reduction under anaerobic conditions.

The metabolites produced from these reactions can either be mere intermediates which have relatively little reactivity towards cellular sysytems and are readily conjugated, or, they can be disruptive to cellular systems. Indeed, inert compounds need to be prepared for conjugation and thus the formation of potentially reactive metabolites is in most cases unavoidable.

There are 57 human CYPs (in 18 families and 42 subfamilies), mostly concentrated in the endoplasmic reticulum of liver cells although many tissues have them to some extent (Nelson DR et al, 2004). CYPs are grouped into 14 families according to their sequence similarity. Generally, enzymes in the same family share 40% sequence similarity and 55% within a subfamily. Nomenclature of P450s is as follows. CYP (to represent cytochrome P450 superfamily), followed by an arabic number for the family, a capital letter for the subfamily and finally a second arabic number to denote the isoenzyme. An example is CYP1A1 which is the first enzyme in subfamily A of cytochrome P450 family 1.

The majority of the enzymes are present in the CYP1-4 families. CYP1-3 are primarily concerned with xenobiotic biotransformation whereas the other CYPs deal primarily with endogenous compounds. The CYP section is structured by the typical substrate they act upon. Of the 57 human CYPs, 7 encode mitochondrial enzymes, all involved in sterol biosynthesis. Of the remaining 50 microsomal enzymes, 20 act upon endogenous compounds, 15 on xenobiotics and 15 are the so-called "orphan enzymes" with no substrate identified.

The P450 catalytic cycle (picture) shows the steps involved when a substrate binds to the enzyme.

(1) The normal state of a P450 with the iron in its ferric [Fe³⁺] state.

(2) The substrate binds to the enzyme.

(3) The enzyme is reduced to the ferrous [Fe²⁺] state by the addition of an electron from NADPH cytochrome P450 reductase. The bound substrate facilitates this process.

(4,5) Molecular oxygen binds and forms an Fe²⁺OOH complex with the addition of a proton and a second donation of an electron from either NADPH cytochrome P450 reductase or cytochrome b5. A second proton cleaves the Fe²⁺OOH complex to form water.

(6) An unstable [FeO]³⁺ complex donates its oxygen to the substrate (7). The oxidised substrate is released and the enzyme returns to its initial state (1).

The very nature of many chemicals that make them suitable for uptake by these routes, in other words their lipophilicty (favours fat solubility) is also the main reason organisms have developed mechanisms that convert them to hydrophilic (favours water solubility) compounds which are readily excreted via bile and urine. Otherwise, lipophilic chemicals would accumulate in the body and overwhelm defense mechanisms. This process is called biotransformation and is catalyzed by enzymes mainly in the liver of higher organisms but a number of other organs have considerable ability to process xenobiotica such as kidneys, gut and lungs. As well as xenobiotics, many endogenous compounds are commonly eliminated by this process.

This mechanism is of ancient origin and a major factor for its development in animals is plants. Most animals are plant eaters and thus are subject to a huge variety of chemical compounds which plants produce to stop themselves being eaten. This complex set of enzymes have several features which make them ideal for biotransformation;

(1) metabolites of the parent chemical are usually made more water soluble so it favours rapid excretion via bile and urine

(2) the enzymes possess broad and overlapping specificity to be able to deal with newly exposed chemicals

(3) metabolites of the parent generally don't have adverse biological effects.

In the real world however, all these criteria have exceptions. Many chemicals are transformed into reactive metabolites. In pharmacology, the metabolites of some parent drugs exert the desired pharmacological effect but in the case of polycyclic aromatic hydrocarbons (PAHs), which can undergo epoxidation, it results in the formation of an electrophile which can attack proteins and DNA.

Metabolism of xenobiotica occurs in several steps called Phase 1 (functionalization) and Phase 2 (conjugation). To improve water solubility, a functional group is added to or exposed on the chemical in one or more steps (Phase 1) to which hydrophilic conjugating species can be added (Phase 2). Functional groups can either be electrophilic (epoxides, carbonyl groups) or nucleophilic (hydroxyls, amino and sulfhydryl groups, carboxylic groups) (see picture).

Once chemicals undergo functionalization, the electrophilic or nucleophilic species can be detrimental to biological systems. Electrophiles can react with electron-rich macromolecules such as proteins, DNA and RNA by covalent interaction whilst nucleophiles have the potential to interact with biological receptors. That's why conjugation is so important as it mops up these potentially reactive species.

Many chemicals, when exposed to certain metabolizing enzymes can induce those enzymes, a process called enzyme induction. The effect of this is that these chemicals accelerate their own biotransformation and excretion. The reverse is also true where some chemicals cause enzyme inhibition. Some other factors that alter enzyme levels are sex, age and genetic predisposition. Between species, there can be considerable differences in biotransformation ability which is a problem faced by drug researchers interpreting toxicological results to humans.

At the same time, all of these processes are tightly integrated. Intermediates in reactions of energy generation are starting materials for biosyntheses of amino acids and other compounds, broad-specificity oxidoreductase enzymes can be involved in both detoxification reactions and biosyntheses, and hormone-mediated signaling processes function to coordinate the operation of energy-generating and energy-storing reactions and to couple these to other biosynthetic processes.