As we have seen, despite their common structure and function, CYP substrate specificity varies enormously within the main families of these enzymes. Aside from CYPs that are involved in steroid synthesis and arachidonic acid metabolism, there are only three CYP families which are relevant to humans in terms of drug and toxin biotransformations. These include:
It is believed that 9 out of 10 drugs in use today are metabolized by only five of these isoforms; CYPs 1A2, 2C9, 2C19, 2D6 and 3A4/5. CYP2E1 is interesting mostly from a toxicological perspective and the internal regulation of small hydrophilic molecules. Each CYP has its own broad substrate ‘preferences’, and in some cases they may not be expressed in some individuals at all, or in very low levels.Using immunologically based methods which employ specific antibodies raised to bind to the various CYPs in ELISA and Western Blotting systems, it has been a remarkable achievement that these often extremely similar isoforms have been distinguished, structurally as well as functionally using other assay systems. However, their respective levels of expression in man are often difficult to determine, particularly concerning members of the same family and subfamily.
CYP regulation-transcriptional
As befits such important and complex enzyme systems, there are multiple layers of CYP modulation, which are far from fully understood. Timeframes of control of CYP expression and functionality range from weeks to fractions of a second. An organism may need to build and/or destroy new hormones, such as during the onset of puberty and the regulation of sexual cycles. In addition, xenobiotic ‘!hreat’ molecules such as toxins or drugs must also be biotransformed. All these instances require a sustained increase in the expression of a CYP or CYPs which processes these particular molecules in response to their changing concentrations. This process of specific isoform enzyme induction is discussed and is governed through sensor molecules in the cytoplasm or more usually the nucleus, which then bind to response elements on the genes which code for the particular CYPs. This transcriptional control thus involves the massive upregulation of the CYP expression system, which includes changes in DNA and various types of RNA activity, which results in increased ribosomal protein manufacture. This process is reversible but rather slow, taking place in humans over 1-3 weeks.
It is now established that there are intermediate steps controlling the CYP transcription process. CYP mRNAs possess sites which can bind a number of regulator molecules which can prevent or reduce translation of the protein (CYP2B1, CYP2E1: see below). Indeed, with the CYP1A family, the ratio between CYP mRNA activity and functional protein produced can be up to 100 : 1, whilst with CYP2C9 it is closer to 1 : 1. In addition, a series of quality control ‘chaperone’ molecules exist (such as BAP31) which assist in the normal assembly, folding and delivery of CYPs to the ER. Misfolded or faulty CYPs are ‘rejected’ and degraded. Indeed, molecules like BAP31 also play a role in how CYPs are finally located in the ER, as well as how long they remain anchored and functional. These steps may provide another opportunity for modulation over days rather than weeks.
CYP Regulation-post-translational
Once assembled and operational, it is logical that there should be further apparatus governing CYP which can respond to ‘fine tuning’ necessary in processes which are controlled over hours and minutes rather than days and weeks. In terms of post-translational modification of a finished protein, one of the most important means is phosphorylation of key amino acids and/or peptides which confer selectivity and specificity on an enzyme. Phosphorylation may control enzymatic activity as well as stability and consequent functional capability, rather like changing the ‘sell by date’ on a foodstuff. Several CYPs may be influenced to some extent post-translationally through phosphorylation, such as CYP3A4, CYP2E1 and CYP1A2 and it is likely that most human CYPs are modulated in this way to some extent and this may provide a control mechanism over timeframes of hours and minutes.
As already described, there is also evidence that CYPs may ‘internally’ automatically regulate their own ability to process substrates, by altering the conformations of their access and egress channels, in response to either the number of substrates binding (see testosterone and CYP3A4 in the next section) and/or the changing physicochemical properties of the substrate as it becomes a product during biotransformation. Binding in the general area of the hydrophobic pockets and access channels can accelerate processing of similar substrates actually at the haem active site and some substrates may impede or accelerate the processing of others, which may be linked with endogenous processes. It is also apparent that the degree of coupling between CYPs and their REDOX partners is also highly variable; indeed, CYP3A4 is usually uncoupled until more than one of several similar substrate molecules is bound. This makes sense, as it is wasteful in traffic to keep the engine ‘in gear’ unless you intend to move off reasonably quickly. Overall, these control mechanisms at the enzymatic level may modulate CYP processing over infinitesimal fractions of a second, as required for endogenous processes such as steroid metabolism.
The main human families of CYPs have been extensively studied over the last 15 years and a summary of what is known of these enzymes is given below.
