In carrying out an assay using cultured hepatocytes, you find that addition of hemin (Fe3+ heme) does not have the expected consequence of reduced protoporphyrin IX synthesis. This result suggests that your hepatocytes harbor a mutant form of one of the heme-regulated enzymes of porphyrin biosynthesis. Which of the following represents the likely enzyme?
(A) ALA dehydratase
(B) ALA synthase
© ferrochelatase
(D) heme oxygenase
(E) PBG deaminase