Saturation Kinetics
Definition
Enzyme behaviour at substrate concentrations above the Km, where the rate of metabolism is constant rather than proportional to concentration. Ethanol saturates alcohol dehydrogenase at low millimolar concentrations, which is why an ethanol infusion blocks methanol metabolism.
- Condition
- Substrate concentration above the Km
- Effect
- Metabolism rate becomes constant, not proportional
- Relevant enzyme
- Alcohol dehydrogenase
- Clinical application
- Ethanol infusion blocks methanol metabolism
Common questions
Why does ethanol infusion work as an antidote for methanol poisoning based on this kinetics principle?+
Because ethanol has a much higher affinity for alcohol dehydrogenase than methanol does, administering enough ethanol saturates the enzyme with ethanol first, effectively blocking methanol from being converted into its more toxic metabolites like formic acid while the body clears the methanol unmetabolised.
How does saturation kinetics affect the interpretation of a blood alcohol concentration over time?+
Once ethanol concentration exceeds the level needed to saturate alcohol dehydrogenase, the body eliminates it at a roughly constant rate rather than a rate proportional to how much is present, which is the basis for the near-linear elimination curves used in back-calculation estimates.
Does saturation kinetics apply equally to all toxins metabolised by the same enzyme pathway?+
Only compounds that are substrates for the same saturable enzyme, such as methanol and ethylene glycol competing with ethanol at alcohol dehydrogenase, show this competitive blocking effect; toxins cleared by different pathways are unaffected by ethanol saturation.
Related terms
- Anion Gap
- Sodium minus the sum of chloride and bicarbonate in serum. A widened anion gap (above 16) in a comatose patient with unexplained...
- Carboxyhaemoglobin (COHb)
- The compound formed when carbon monoxide binds to haemoglobin at the oxygen-binding site, with an affinity 200-250 times greater than oxygen. Produces...
- Cherry-Pink Lividity
- The bright pink discolouration of dependent tissues seen in CO and cyanide deaths. In CO the colour comes from COHb itself, in...
- Conway Microdiffusion
- A two-chamber glass cell in which an acid in the outer ring liberates HCN from the sample and the gas diffuses into...
- Headspace GC
- Gas chromatography in which the sample (blood, urine, vitreous) is sealed in a vial, equilibrated at a fixed temperature, and a portion...