CASE HISTORY • Part 1
A 45-year-old housepainter presents with several weeks of worsening abdominal pain, headache, insomnia, and difficulty gripping his paint brush. His past history and review of systems (ROS) is positive for smoking, alcoholism, and reactive depression. Vital signs: BP - 165/95 mm Hg, P - 84 bpm, R - 16 bpm. Examination is positive for grayish staining of the gum line, hepatomegaly, diffuse abdominal tenderness without rebound, and 2+ weakness of wrist extensors bilaterally.
CBC: Hematocrit/hemoglobin - 36%/12 g/dL
MCV - 87 fL MCH - 30 pg MCHC - 31 g/dL
RDW-CV - 12%
WBC count - 6,500/μL
Platelet count - 160,000/μL SMEAR MORPHOLOGY
Normocytic and normochromic red cells, a few hypochromic cells, and occasional "shift cells" with questionable stippling. Normal white blood cell and platelet morphology.
Reticulocyte count/index - 4%/1.5–2.0 Questions
The porphyrias are caused by inherited defects in the heme biosynthetic pathway that result in excess production of porphyrin precursors. Depending on the type of excess porphyrin produced, patients can experience severe photosensitivity, nerve damage, liver disease, and anemia. Detection requires a high level of suspicion and skill in using the laboratory. Accurate diagnosis is important because there are now effective therapies for several of these defects.
NORMAL PORPHYRIN SYNTHESIS
The normal heme biosynthetic pathway is shown in Figure 14-1. The initial step is condensation of glycine and succinyl-CoA to form δ-aminolevulinic acid (δ-ALA), which is catalyzed by a mitochondrial enzyme, ALA synthase. This is the only enzyme step that requires a cofactor, in this case, pyridoxal-5-phosphate (vitamin B6).
The normal heme biosynthetic pathway. The first step in the synthesis of heme is the condensation of glycine and succinyl-CoA with pyridoxal 5-phosphate as the coenzyme to form δ-aminolevulinic acid (δ-ALA). This process is carried out in mitochondria catalyzed by the enzyme ALA synthase. The subsequent formation of porphobilinogen, uroporphyrinogen III, and coproporphyrinogen III occurs in the cell cytoplasm. The final steps of protoporphyrin production and the assembly of protoporphyrin IX and iron to form heme depend on mitochondrial enzymes.
The next several steps include formation of porphobilinogen, uroporphyrinogen III, and coproporphyrinogen III, which are catalyzed by cytosolic enzymes. The final three steps in the sequence depend on mitochondrial enzymes (coproporphyrinogen oxidase, protoporphyrinogen oxidase, and ferrochelatase). Chelation of iron to protoporphyrin IX depends on an adequate supply of iron; deficient patients accumulate excess protoporphyrin in their red blood cells.
Porphyrin synthesis is regulated by the activity of ALA synthase and the end product, heme. Levels of transcription and translation of ALA synthase in mitochondria are a function of the heme concentration. This ...