How is melatonin metabolized?

Melatonin Metabolism Pathway

Melatonin is primarily metabolized in the liver through hydroxylation by cytochrome P450 enzymes, with CYP1A2 being the main enzyme responsible for converting melatonin to 6-hydroxymelatonin, which is then conjugated with sulfate and excreted in urine as 6-sulfatoxymelatonin. 1, 2

Primary Metabolic Pathway

The metabolism of melatonin follows a clear enzymatic pathway:

1. Hepatic Metabolism via CYP450 Enzymes:

   • CYP1A2 is the principal enzyme responsible for melatonin metabolism in the liver 1
   • CYP1A1 and CYP1B1 also contribute to melatonin 6-hydroxylation 2
   • CYP2C19 plays a minor role in both 6-hydroxylation and O-demethylation 2

2. Main Metabolic Reactions:

   • 6-Hydroxylation: The dominant pathway, converting melatonin to 6-hydroxymelatonin
   • O-Demethylation: A minor pathway producing N-acetylserotonin, primarily mediated by CYP2C19 and to some extent CYP1A2 2

3. Phase II Metabolism:

   • 6-Hydroxymelatonin undergoes conjugation with sulfate
   • The resulting metabolite, 6-sulfatoxymelatonin (aMT6s), is the primary urinary metabolite of melatonin 3

Enzyme Kinetics and Specificity

The affinity and catalytic efficiency of the CYP enzymes for melatonin metabolism vary:

• CYP1A2: Km = 25.9 ± 2.47 μM, Vmax = 10.6 ± 0.32 pmol/min/pmol P450 2
• CYP1A1: Km = 19.2 ± 2.01 μM, Vmax = 6.46 ± 0.22 pmol/min/pmol P450 2
• CYP1B1: Km = 30.9 ± 3.76 μM, Vmax = 5.31 ± 0.21 pmol/min/pmol P450 2

CYP1A2 shows the highest catalytic efficiency (Vmax/Km ratio) for melatonin metabolism, confirming its role as the primary enzyme in this process 2.

Extrahepatic Metabolism

While the liver is the main site of melatonin metabolism, extrahepatic metabolism also occurs:

• CYP1B1 is expressed in various extrahepatic tissues including the intestine and cerebral cortex 2
• This enzyme contributes to local regulation of melatonin and 6-hydroxymelatonin levels in these tissues 2
• Brain homogenates from studies comparing wild-type and cyp1b1-null mice confirm CYP1B1's significant role in melatonin 6-hydroxylation 2

Clinical Implications and Drug Interactions

The metabolism of melatonin by CYP enzymes has important clinical implications:

• Drug Interactions: Medications that inhibit CYP1A2 can increase melatonin levels

  • Fluvoxamine is a potent inhibitor of 6-hydroxymelatonin formation, giving 50-69% inhibition at concentrations of 1-10 μM 1
  • Other inhibitors include furafylline, sulphaphenazole, and omeprazole 1

• Potential as a Probe Drug: Melatonin has been investigated as a potential probe drug for CYP1A2 activity assessment

  • A significant correlation exists between apparent clearance of melatonin and caffeine clearance (Rs = 0.75, P = 0.005) 4

Measurement and Assessment

When measuring melatonin metabolism:

• The primary urinary metabolite, 6-sulfatoxymelatonin (aMT6s), can be assessed to evaluate melatonin metabolism 3, 5
• Collection protocols are critical for accurate assessment:
  • Avoid NSAIDs for 72 hours before collection
  • Avoid caffeine and alcohol for 24 hours before collection
  • Maintain consistent posture and avoid exercise during collection 5
  • Collect samples under dim light conditions (<5 lux) 3, 5

Pitfalls and Caveats

Several factors can affect melatonin metabolism assessment:

1. Timing Variability: Melatonin levels naturally fluctuate throughout the day, with highest levels at night 5

2. Environmental Factors: High altitude exposure can increase melatonin metabolite levels 5

3. Medication Effects:

   • NSAIDs, beta-blockers, and alcohol can suppress melatonin production 3
   • CYP1A2 inhibitors can significantly alter melatonin metabolism 1

4. Individual Variability: Despite CYP1A2's importance in melatonin metabolism, studies have not found a clear inverse correlation between CYP1A2 activity and endogenous melatonin levels, suggesting compensatory mechanisms may exist 6

5. Collection Methodology: Improper collection techniques can significantly affect results, leading to misinterpretation 5

Multiple assessments are recommended to establish abnormal patterns, as single measurements can lead to incorrect conclusions 5.