What happens when two substances, both substrates for Cytochrome P450 metabolism, are both present in the bloodstream?

For example, with sertraline and cannabidiol (CBD), if someone took a sertraline tablet and later used CBD, how does the presence of two CYP substrates affect drug metabolism? Does one drug inhibit the metabolism of the other? What happens to blood levels of the drugs? Does the order of administration matter?

  • $\begingroup$ If you want to ask the question about two specific substances and one specific enzyme, you should edit the title of your question to make that clear. If you are asking about the broader biochemical principle then you should edit the body of your question to make that clear. $\endgroup$
    – Bryan Krause
    Commented Jan 29, 2019 at 16:24
  • $\begingroup$ Welcome to SE Biology. As a new user you should take the tour and read the help on asking good questions, where you will see that you are expected to demonstrate that you have researched a question before posting. This question can be answered by an understanding of the basic principles of enzyme action. Read, for example, this section of Berg et al. online, paying particular attention to the meaning of Km. Oh, and what is CBD? Don't tell me, edit your question. $\endgroup$
    – David
    Commented Jan 29, 2019 at 16:41
  • $\begingroup$ @David Km? I think you're overly simplifying this question, as if you are pointing someone asking about events at relativistic speeds towards Newton's laws. The whole point is that this is a case where MM kinetics don't necessarily apply (I don't know about the specific example OP references, but I am thinking of liver enzymes like this one ). $\endgroup$
    – Bryan Krause
    Commented Jan 29, 2019 at 16:55
  • $\begingroup$ @BryanKrause The etermal question — what does the poster already know? I would assume from the terminology of "take priority" and "take over" in a question written in reasonable English that the poster is unaware that different substrates for an enzyme can have different affinities. Likewise his reference to x mg of one substrate but an unspecified amount of the second suggests he is unaware of the effect of concentration (and relative concentration of substrate). But I may be wrong. Let him tell us. $\endgroup$
    – David
    Commented Jan 29, 2019 at 17:01
  • 2
    $\begingroup$ @David I agree that the language is a bit sloppy, but it seems like OP is coming at this with some knowledge of medical implications of pharmacology rather than biochemistry and I think this question is better researched than you are giving credit for. $\endgroup$
    – Bryan Krause
    Commented Jan 29, 2019 at 17:04

2 Answers 2


I think the OP is asking the following:

If an enzyme is not specific one substrate, but is simultaneously presented with a pair of substrates, which one 'wins' and how do we decide?

For example, if an enzyme is presented with a racemate such that the D-form has a 10-fold lower $K_m$ than the L-form, but the L-form has a 5-fold higher $k_{cat}$, which one is preferentially metabolized, and how do we decide?

It was Fersht who first provided the definitive answer, at least for single-substrate enzymes (Fersht, Enzyme Structure and Mechanism, 1977; see also here)

The kinetic parameter determining the specificity for competing substrates is neither $K_m$ nor $k_{cat}$, but the (so-called) specificity constant: $$\frac{k_{cat}}{K_m}$$

If it assumed that both substrates, considered in isolation, obey the Michaelis-Menten kinetics, then it is very easy to derive (starting with the ratio of the respective Michaelis-Menten equations) the following expression for the initial velocities when both substrates are present simultaneously.

$$ { {{{v_{i}^1}}\over{v_{i}^2}}} = {{{{k_{cat}^1}}\over{K_{m}^1}[S_1]}} \times{{{{K_{m}^2}[S_2]}\over{k_{cat}^2}}} \ \ \ \ \ (1)$$

(The equation is derived by, among others, Cornish-Bowden (2004, pp 36-39):

If $V_{max}$ is preferred to $k_{cat}$: $$ { {{{v_{i}^1}}\over{v_{i}^2}}} = {{{{V_{max}^1}}\over{K_{m}^1}[S_1]}} \times{{{{K_{m}^2}[S_2]}\over{V_{max}^2}}} \ \ \ \ \ (2)$$

${{k_{cat}}}\over{K_m}$ has units of a second-order rate constant and, giving it a symbol (${k_{1}^{\prime}}$ for $S_1$ and ${k_{2}^{\prime}}$ for $S_2$) to reflect its status as a fundamental kinetic constant and not just a ratio, allows the following equation to be written:

$$ { {{{v_{i}^1}}\over{v_{i}^2}}} = {{{{k_{1}^{\prime}}}\over[S_1]}} \times{{{[S_2]}\over{k_{2}^{\prime}}}} \ \ \ \ \ (3)$$

This equation provides the answer to the OP question. For the racemate described above, we need only calculate the ratio of the specificity constants to get the answer:

$${ { { {v_{i}^{D_{form}} } }\over{ v_{i}^{L_{form}}} } } = { { { {k_{D_{form}}^{\prime}\times{[D_{form}]} } }\over{ k_{L_{form}}^{\prime}}\times{[L_{form}]} } } = { { { {k_{D_{form}}^{\prime} } }\over{ k_{L_{form}}^{\prime}} } } = 2 \ \ \ \ \ (4)$$

Much could be written about the specificity constant (diffusion control, enzyme 'perfection', Haldane relationships), but I'll make just one. Considering the single-substrate Michaelis-Menten enzyme, the following holds AT ALL SUBSTRATE CONCENTRATIONS

$$ { {{{v_{i}}}}} = {{{{k_{1}^{\prime}}}}[{S_o}][E]} \ \ \ \ \ (5)$$

When $S_o$ is very low, $[E]$ = $e_o$ (approx) and the following may be written (where,unlike $[E]$, $e_o$ is usually known or may be determined) $$ { {{{v_{i}}}}} = {{{{k_{1}^{\prime}}}}[{S_o}]e_o} \ \ \ \ \ (6)$$

But it is Eqn (5) that is the important one: $k^{\prime}$ (the so-called specificity constant) may be thought of as the second-order rate constant governing the combination of ('free') enzyme with substrate at all substrate concentrations, not just when $S_o$ << $K_m$

Much could be written about this as well


I think it is reasonably obvious what everything means. Nevertheless, here is a list:

  • $K_{m}^1$ is the Michaelis Constant for substrate 1
  • $K_{m}^2$ is the Michaelis Constant for substrate 2

  • $k_{cat}^1 $ is catalytic constant for substrate 1

  • $k_{cat}^2 $ is the catalytic for substrate 2

  • $V_{max}^1 $ is the maximum velocity for substrate 1

  • $V_{max}^2 $ is the maximum velocity for substrate 2

  • $k_{1}^{\prime} $ is the specificity constant for substrate 1

  • $k_{2}^{\prime} $ is the specificity constant for substrate 2

  • $v_{i}^{1} $ is the initial velocity due to substrate 1

  • $v_{i}^{2} $ is the initial velocity due to substrate 2


  • Cornish-Bowden (2004) Fundamentals of Enzyme Kinetics 3rd Edn. Portland Press (London)
  • $\begingroup$ You really think? If only questions on SE Biology were at that level. To avoid your answer going to waste I’d edit the question drastically so there is no doubt your answer fits. If the poster doesn’t like it he can always revert, but his lack of response to comments suggests this unlikely. $\endgroup$
    – David
    Commented Mar 2, 2019 at 14:12
  • $\begingroup$ I enjoyed this answer. I'd be curious to see your thoughts about how well this applies to drug metabolizing CYPs in vivo, given their particular properties. $\endgroup$
    – De Novo
    Commented Mar 2, 2019 at 14:24
  • $\begingroup$ FYI I've edited the question to, I think, let the useful pharmacology question be more clear. Along with @David, I'd encourage you to edit the question as well, since you saw a useful biochemistry question in it. $\endgroup$
    – De Novo
    Commented Mar 2, 2019 at 18:40

What are CYPs? How are they different from other enzymes?

Cytochrome P450 enzymes (CYPs) are a large family of enzymes involved in the metabolism of many endogenous and exogenous compounds (including typical dietary substances and pharmaceuticals). There are many different CYPs, each individual CYP form can metabolize many different compounds, and a single compound is typically metabolized by more than one CYP, sometimes at the same site, sometimes at multiple sites, sometimes in an ordered sequence, and sometimes not. This promiscuity in substrate/enzyme interaction is in contrast to most human enzymes, which are highly specific. Its cost, though, is efficiency.

CYPs and drug-drug interactions

CYP effects are an important subset of drug-drug interactions, but this is typically not described in terms of race conditions. Perhaps this is because of the particular relationship between enzyme and substrate. In addition to being metabolized by CYPs, many drugs and dietary substances can inhibit some CYP forms and induce others (including forms that do not metabolize the substance itself).

To answer the particular question about whether the order of administration matters, while inhibition is typically at the level of protein interaction, induction involves gene interactions. Because induction involves gene interactions, induction results in interactions that do not depend on both drugs being available to CYP at the same time. This means that (for induction effects) neither changing the order of administration or staggering doses will prevent the interaction. Generally, those examples of drug-drug interaction that are readily prevented by dose order involve pharmacodynamic interactions, i.e., interactions at the drug's intended molecular target.

Is this all you have to consider when evaluating for a potential drug-drug interaction?

For emphasis, I'll say it again: CYP effects are an important subset of drug-drug interactions. There are many things to consider when evaluating the effect of polypharmacy.

Further reading

You can read about this in Goodman & Gilman, the Pharmacological Basis of Therapeutics. Ch 6 has a good subsection about CYP metabolism in general, with specific discussion about the metabolism of different compounds in other chapters. Most good basic pharmacology textbooks will cover this, but I think G&G does a particularly good job. Full prescribers information on any drug will include information on CYP450 metabolism, inhibition, and induction. You can look at wikipedia, but this is not the best wikipedia article I've read.

So what about sertraline and CBD?

I am not going to make a statement about whether a particular combination of drugs does or doesn't interact, if only to encourage anyone with a drug-drug interaction question TALK TO THEIR DOCTOR, or whoever prescribed your medication. Different jurisdictions have different laws about CBD, but doctors are not law enforcement officials. You can and should tell him/her about everything you take.

  • $\begingroup$ If you think that this is a personal medical question (that hadn’t occurred to me), surely you should vote to close. $\endgroup$
    – David
    Commented Mar 2, 2019 at 17:12
  • $\begingroup$ @David as, alas, we often see, it seems to me like there is one question in the text of the OP/post (not personal medical) and another question in the mind of the OP/poster, but the OP/poster hasn't engaged with the comments. I tried to answer the text of the question, and only because there was excellent information in another answer. $\endgroup$
    – De Novo
    Commented Mar 2, 2019 at 17:50

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