Heme synthesis - note that some reactions occur in the cytoplasm and some in the mitochondrion (yellow)
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Erythropoietic protoporphyria

The porphyrias are inherited or acquired disorders of certain enzymes in the heme biosynthetic pathway (also called porphyrin pathway). They are broadly classified as hepatic porphyrias or erythropoietic porphyrias, based on the site of the overproduction and mainly accumulation of the porphyrins (or their chemical precursors). more...

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Overview

In humans, porphyrins are the main precursors of heme, an essential constituent of hemoglobin, myoglobin, and cytochrome.

Deficiency in the enzymes of the porphyrin pathway leads to insufficient production of heme. This is, however, not the main problem; most enzymes—even when less functional—have enough residual activity to assist in heme biosynthesis. The largest problem in these deficiencies is the accumulation of porphyrins, the heme precursors, which are toxic to tissue in high concentrations. The chemical properties of these intermediates determine in which tissue they accumulate, whether they are photosensitive, and how the compound is excreted (in the urine or feces).

Subtypes

There are eight enzymes in the heme biosynthetic pathway: the first and the last three are in the mitochondria, while the other four are in the cytosol.

  1. δ-aminolevulinate (ALA) synthase
  2. δ-aminolevulinate (ALA) dehydratase
  3. hydroxymethylbilane (HMB) synthase
  4. uroporphyrinogen (URO) synthase
  5. uroporphyrinogen (URO) decarboxylase
  6. coproporphyrinogen (COPRO) oxidase
  7. protoporphyrinogen (PROTO) oxidase
  8. ferrochelastase

Hepatic porphyrias

The hepatic porphyrias include:

  • ALA dehydratase deficiency
  • acute intermittent porphyria (AIP): a deficiency in HMB synthase
  • hereditary coproporphyria (HCP): a deficiency in COPRO oxidase
  • variegate porphyria (VP): a deficiency in PROTO oxidase
  • porphyria cutanea tarda (PCT): a deficiency in URO decarboxylase

Erythropoietic porphyria

The erythropoietic porphyrias include:

  • X-linked sideroblastic anemia (XLSA): a deficiency in ALA synthase
  • congenital erythropoietic porphyria (CEP): a deficiency in URO synthase
  • erythropoietic protoporphyria (EPP): a deficiency in ferrochelatase

Porphyria variegata

Variegate porphyria (also porphyria variegata or mixed porphyria) results from a partial deficiency in PROTO oxidase, manifesting itself with skin lesions similar to those of porphyria cutanea tarda combined with acute neurologic attacks. It may first occur in the second decade of life; there is a cohort of sufferers living in South Africa descended from a single person from the Netherlands, Berrit Janisz, who emigrated in the 17th century.

Signs and symptoms

The hepatic porphyrias primarily affect the nervous system, resulting in abdominal pain, vomiting, acute neuropathy, seizures, and mental disturbances, including hallucinations, depression, anxiety, and paranoia. Cardiac arrhythmias and tachycardia (fast heart rate) may develop as the autonomic nervous system is affected. Pain can be severe and can, in some cases, be both acute and chronic in nature. Constipation is frequently present, as the nervous system of the gut is affected.

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Skin redness following minimal sun exposure: immediate erythema and edema appear at sites unprotected by clothing - Derm DDx - Brief Article
From Geriatrics, 12/1/03 by Norman Levine

63-year-old woman presents with a history of immediate skin redness following minimal sun exposure. The erythema fades in less than 24 hours. She has a history of hypertension and is on lisinopril and hydrochlorothiazide. On initial examination, the skin appears normal. After being sent outdoors for 15 minutes, the patient returns with skin that appears red and mildly edematous at sites that are unprotected by clothing (figures 1 and 2).

[FIGURES 1 & 2 OMITTED]

Differential diagnosis

Conditions to consider are those that produce an immediate sun-induced eruption. Sunburn looks almost identical to this patient's condition. However, it takes at least 12 hours after exposure to manifest the characteristic findings.

Drug-induced photosensitivity reaction has the appearance of exaggerated sunburn, as in this case, but it takes at least 24 to 48 hours after exposure to develop the cutaneous signs.

Erythropoietic protoporphyria is an autosomal dominant disease in which an abnormal heme byproduct is deposited in the skin and may produce immediate signs and symptoms of photosensitivity. However, patients usually manifest the disease early in life and have a burning sensation associated with the erythema.

Solar urticaria is the correct diagnosis. This presumed allergic reaction to an unknown inciting agent produces almost immediate erythema and edema of the skin. It may arise for the first time at any age and lasts indefinitely. The only completely effective therapy is sun avoidance.

The patient is being treated indefinitely with doxepin, 25 mg, at night, and she was advised to avoid all unnecessary sun exposure. She continues to develop new lesions on a regular basis, which occurs in most patients with this condition.

Diagnostic pearl

Consider solar urticaria in cases of immediate sun-sensitivity reactions.

Dr. Levine is professor of medicine (dermatology), University of Arizona Health Sciences Center, Tucson.

COPYRIGHT 2003 Advanstar Communications, Inc.
COPYRIGHT 2004 Gale Group

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