The two main sites of production of heme are

• A. Liver and bone marrow
• B. Heart and lung
• C. Muscle and blood
• D. Liver and spleen

Answer: (A)

Heme biosynthesis happens where there is the greatest need for heme-containing proteins: in bone marrow for red blood cell (erythroid) development and in the liver for numerous heme enzymes. In the bone marrow, developing erythroblasts assemble heme to combine with globin to make hemoglobin, driven by high activity of the erythroid-specific enzyme ALA synthase 2 and the abundant mitochondria necessary for the pathway’s early steps. In the liver, hepatocytes supply heme for cytochromes and other enzymes involved in metabolism and detoxification, regulated by the hepatic enzyme ALA synthase 1 and by feedback from cellular heme levels. While other tissues do conduct some heme synthesis, they do not contribute the bulk of the body’s heme, making the two sites the primary sources.