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Inherited disorder of heme biosynthesis.

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See Table P-2.

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Table P-2 Classification of the Inherited Human Porphyrias 
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Inherited enzymatic defects of heme biosynthesis. The heme molecule is thereafter included in the biosynthesis of hemoproteins such as hemoglobin, myoglobin, microsomal cytochrome P450, peroxidase, and cyclooxygenase. They are classified as erythropoietic or hepatic depending on the primary organ in which excess production of porphyrins or precursors occurs (see Table P-2).

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Acute intermittent porphyria (AIP) 1:20,000 in Europe, 1:10,000 in Sweden; variegate porphyria (VP) 1:250 to 1:500 in white South Africans; erythropoietic protoporphyria (EPP) 1:35,000 live births.

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See the classification Table P-2.

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Overproduction of specific heme precursors occurs upstream the characteristic enzymatic defect. This leads to increased baseline aminolevulinic acid (ALA) synthetase activity. The first and last three enzymes involved in heme synthesis are located in the mitochondria. The others are in the cytosol. Disease manifestations may be a result of increased ALA synthetase activity, increased porphyrin accumulation, or decreased heme production. Porphyrins are highly reactive oxidants, and skin accumulation causes photosensitivity. Neuronal damage throughout the central and peripheral nervous system also occurs, though the mechanism of this severe neuropathy is unknown. See Table P-3.

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Table P-3 Pathophysiology of Porphyrias and Enzyme Defects 
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Difficult in the latent stage, but diagnosis is possible by DNA analysis. The presence of urine that becomes dark or port-wine reddish color after staying should prompt a diagnosis workup. During a porphyric crisis, measurement of urine porphyrin and porphyrinogen precursors is indicated and will confirm the diagnosis.

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Acute attacks occur in AIP, hereditary coproporphyria (HCP), VP, and Doss porphyria (DP). These attacks are often triggered by certain drugs, although these drugs (see Pharmacological Implications and Table P-4 below) do not always trigger attacks, as well as by fasting, alcohol intake, or infection. Although hepatic porphyrias are rarely symptomatic before ...

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