Abnormal tissue enzyme concentration or activity Low tissue ferrochelatase activity 2024-04-04T08:15:27Z Concentration or activity of Ferrochelatase (FECH; EC 4.99.1.1) below the lower limit of normal. FECH enzyme can be measured in multiple tissues including leukocytes and cultured fibroblasts. FECH is the terminal enzyme of the heme biosynthetic pathway, and catalyzes the insertion of iron into protoporphyrin to form heme.