How CYP isoforms operate in vivo (How Oxidative Systems Metabolize Substrates) (Human Drug Metabolism)

The detailed processes on how living systems operate are sometimes focused on at the expense of a global understanding of how these systems might operate in the tissue. It is useful to try to visualize how CYPs process massive numbers of molecules from hydrophobic to at least partially hydrophilic products every second. If you visualize just one hepatocyte, and imagine the smooth endoplasmic reticulum, with its massive surface area, with vast numbers of CYP and REDOX partners embedded in its tubing, then you can see how the liver can sometimes metabolize the majority of drugs and endogenous substrates in a given volume of blood in just one passage through the organ, rather like a automotive catalytic converter forms water and CO2 and nitrogen oxides from a hundreds of combustion products.

Illustrative use of structures

Most topics at this point show a large number of chemical reactions that highlight how CYP isoforms metabolize specific drugs/toxins/steroids, etc and this one is no exception. However, it is appreciated that many students might not have studied chemistry, or struggle with it as a subject and feel intimidated by chemical structures. This is worth overcoming, as some qualitative understanding of basic organic chemistry really pays off in illustrating, understanding and appreciating how CYP enzyme systems operate at the molecular level. If you study the diagrams it should not be too difficult to eventually see a molecule in a way that approaches how a CYP enzyme might ‘see’ it. After all, it is worth the effort, as we thrive as a species in part due to these remarkable enzymes.

Primary purposes of CYPs

As mentioned before, CYP isoforms have evolved to:

• make a molecule less lipophilic (and often less stable) as rapidly as possible;

• make some molecules more vulnerable to conjugation.

The first step is the binding of the substrate. As you will have seen, individual CYPs bind groups of very broadly similar chemical structures. This is partly achieved by the size and physicochemical characteristics of the molecule, as we have seen. For example, the entrance to CYP2C9 is not wide enough to bind part of a large molecule like cyclosporine, so this molecule is virtually excluded from all the CYPs, except the one with the largest and most flexible entrance and binding area, CYP3A4. The processes involved in the orientation of substrates to proximity with the haem iron are complex as mentioned previously, but once an agent is presented to the iron, oxidation and occasionally reduction, can then occur.

Role of oxidation

CYP metabolism is almost always some form of oxidation, which can achieve their main aims. Oxidizing a molecule can have three main effects on it, as follows.

Increase in hydrophilicity

Forming a simple alcohol or phenol is often carried out to make a molecule soluble in water so it can be eliminated without the need for any further metabolic input.

Reduction in stability leading to structural rearrangement

Obviously some chemical structures are inherently less stable than others and any prototype drugs that are unstable and have the potential to react with cellular structures are weeded out in the drug discovery process. However, the process of CYP-mediated metabolism, where a stable drug is structurally changed, can form a much more reactive and potentially toxic product.A very young child hitting objects randomly with a piece of metal will not be able to discern the difference between an inert object and an extremely dangerous one (electrical equipment or an explosive device). In the same way, a molecule may be bound and metabolized by CYPs, irrespective of the impact these processes may have on the stability and potential toxicity of the product. There is a risk that the new molecule may be very reactive and dangerous indeed and may attack the CYP itself or the surrounding cellular structures. Although this does happen, evolution has retained the advantages of CYPs, such as their ability to process virtually any required molecule, through the appearance of conjugation and detoxification systems that contain and usually quench the reactivity of these agents. This could be compared with the evolution of the Porsche 911. The weight of the engine over the driven rear-wheels offers tremendous traction and thus acceleration. However, intensive modification of the car over many years has counteracted the 911’s inherent tendency to carry straight on through corners and vastly increased its safety whilst retaining its performance. With CYP oxidation processes, the evolution of attendant detoxification systems ensures that the risk to the cell of creating a reactive species usually pays off and a molecule can be quite radically changed in terms of its physicochemical properties without problems. For example, a lipophilic functional group might be oxidized to an alcohol, which may be so unstable that it breaks off. This has the dual advantage of removing a lipophilic structure that leaves the molecule more hydrophilic (see the oxidation of terfenadine). It can also pave the way for further metabolism, such as conjugation.

Facilitation for c onjugation

Many oxidative metabolites are much more vulnerable than their parent molecules to reaction with water-soluble groups such as glucuronic acid and sulphates. Once a conjugate is formed, this vastly improves water solubility and Phase III transport systems will generally remove it from the cell and into the blood.

Summary of CYP operations

A sculptor was once asked how he would go about sculpting an elephant from a block of stone. His response was ‘knock off all the bits that did not look like an elephant’. Similarly, drug-metabolizing CYPs have one main imperative, to make molecules more water-soluble. Every aspect of their structure and function, their position in the liver, their initial selection of substrate, binding, substrate orientation and catalytic cycling, is intended to accomplish this deceptively simple aim.

With experience, you should be able to look at any drug or chemical and make a reasonable stab at suggesting how a CYP enzyme might metabolize it. It is important to see these enzymes not as carrying out thousands of different reactions, but as basically carrying out only two or three basic operations on thousands of different molecules every second.

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