Delta-aminolevulinic acid synthase (ALAS) is the rate-limiting enzyme of heme synthesis and is feedback-inhibited by heme. In the liver, ALAS1 (housekeeping) mRNA contains an iron-responsive element (IRE) in the 5'-UTR. When iron stores are high, iron regulatory protein (IRP) releases the IRE, allowing translation. How does glucose (high carbohydrate intake) suppress hepatic ALAS1 expression in acute porphyria?

• A Glucose reduces the transcription factor PGC-1alpha, which normally co-activates ALAS1 gene transcription through nuclear receptor binding to the ALAS1 promoter ✓
• B Glucose increases heme synthesis, providing product feedback inhibition on ALAS1
• C Glucose activates AMPK, which phosphorylates ALAS1 to reduce its mitochondrial import
• D Glucose reduces hepatic cytochrome P450 induction, indirectly decreasing heme demand

Explanation

The 'glucose effect' in acute porphyria treatment (IV dextrose or high carbohydrate diet) works through suppression of PGC-1alpha (peroxisome proliferator-activated receptor gamma coactivator-1alpha). Under fasting/starvation conditions, glucagon and low glucose activate PGC-1alpha, which co-activates nuclear receptor 4A (NR4A) and other transcription factors at the ALAS1 gene promoter, upregulating ALAS1 transcription and precipitating porphyria attacks. Glucose (fed state) suppresses PGC-1alpha activity, reducing ALAS1 transcription. This mechanistically explains why fasting precipitates attacks and glucose IV is therapeutic. Hemin (IV heme) is more effective because it directly feedback-inhibits ALAS1. AMPK is activated by low energy/fasting, not high glucose.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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Written and medically reviewed by the StethoPrep medical team.