Substrate

Substrate, in pharmacology, refers to the molecule that an enzyme or transporter acts upon — the chemical being processed, as distinct from inhibitors or inducers, which alter how fast that processing runs. When a drug is described as a substrate of an enzyme, it means the enzyme transforms that drug into one or more metabolites.

Most of the body's drug metabolism occurs in the liver and gut, where enzyme families convert substrates into more water-soluble forms that are easier to excrete. Transporters also have substrates — molecules they carry across cell membranes — which determines how much of a drug reaches its target tissue and, in the case of the blood-brain barrier, how much enters the CNS.

How it works · its role

The cytochrome P450 family (CYP enzymes) handles the majority of psychoactive drug metabolism. Each enzyme in the family recognises a characteristic set of substrates and oxidises them. CYP3A4 processes the largest proportion of drugs in clinical use; CYP2D6 is especially relevant to psychoactives, metabolising many opioids, antidepressants, and stimulants.

When a substrate binds to the enzyme's active site, the enzyme converts it — typically into a more water-soluble form. The resulting metabolites may be pharmacologically inactive, active in their own right, or in some cases more toxic than the parent compound. That conversion rate governs the drug's half-life and the plasma concentration a given dose produces.

Genetic variation in these enzymes matters considerably. Roughly 7–10% of people of European descent carry non-functional copies of the CYP2D6 gene, making them poor metabolisers: any CYP2D6 substrate clears far more slowly in them, and a standard dose can behave like a substantially larger one.

Relevance to substances & effects

Substrate status is the mechanism behind most metabolic drug interactions. When two substrates share the same enzyme, they compete for binding — each clears more slowly than it would alone, pushing plasma levels higher. Adding an inhibitor of that enzyme slows clearance further still.

Many well-known psychoactives are CYP2D6 substrates. Codeine must be converted by CYP2D6 to become morphine; in poor metabolisers that conversion barely happens, so a standard dose produces little pain suppression.

MDMA is extensively metabolised by CYP2D6 and inhibits it in the process, limiting its own clearance at typical doses — an effect that collapses under high doses or when other inhibitors are present. Many SSRIs are both substrates and inhibitors of CYP2D6, which is why they recur throughout interaction tables.

Transporters follow a parallel logic. P-glycoprotein, an efflux pump found at the gut wall and blood-brain barrier, actively expels drugs it recognises as substrates, reducing oral bioavailability and limiting brain penetration. Some cannabinoids and opioids reach the CNS at lower levels than their chemistry alone would predict, partly because of P-glycoprotein substrate relationships.