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. Author manuscript; available in PMC: 2017 Sep 1.
Published in final edited form as: Curr Dermatol Rep. 2016 Jul 13;5(3):179–190. doi: 10.1007/s13671-016-0154-5

Fig. 1.

Fig. 1

Heme biosynthetic cycle in eukaryotic cells. Exogenously administered ALA bypasses the feedback control mechanism whereby heme inhibits ALA synthase which is designed to prevent excess accumulation of PPIX in normal cells. Consequently, excess free PPIX accumulates in the cells since ferrochelatase is now rate limiting. ALA-S 5-aminolevulinic acid synthase, ALA-D 5-aminolevulinic acid dehydratase, PBG-D porphobilinogen deaminase, UCS uroporphyrinogen co-synthase, UGD uroporphyrinogen decarboxylase, CPO coproporphyrinogen oxidase, PPO protoporphyrinogen oxidase, FCH ferrochelatase