A 29-year-old woman in an acute intermittent porphyric crisis receives intravenous hemin (heme arginate) along with supportive care. By what mechanism does hemin terminate the attack?

• A It chelates accumulated delta-aminolevulinic acid in plasma
• B It induces cytochrome P450 enzymes to consume excess porphyrin precursors
• C It replaces the defective porphobilinogen deaminase enzyme activity
• D It represses transcription of hepatic ALA synthase 1, reducing ALA and porphobilinogen production ✓

Explanation

Exogenous heme restores the intracellular free heme pool, which feeds back on the heme-regulated machinery and suppresses ALAS1 mRNA synthesis and its transfer into mitochondria. With less ALA formed, levels of the neurotoxic precursors ALA and porphobilinogen fall and symptoms resolve. Hemin does not replace PBG deaminase and does not induce P450; inducing P450 would increase heme consumption and worsen the block, as happens with barbiturates.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.