Clinical meaning
The porphyrias are a group of metabolic disorders caused by inherited or acquired deficiencies in specific enzymes of the heme biosynthetic pathway, resulting in accumulation and excess excretion of porphyrins or porphyrin precursors. Heme synthesis is an eight-step enzymatic process beginning with the condensation of glycine and succinyl-CoA by ALA synthase (ALAS) in mitochondria to form delta-aminolevulinic acid (ALA), which is then converted through a series of cytoplasmic and mitochondrial reactions to protoporphyrin IX, which finally chelates iron to form heme. Each enzyme deficiency produces a distinct clinical syndrome classified as either acute hepatic porphyrias or cutaneous porphyrias based on the primary site of porphyrin overproduction and clinical presentation. In acute intermittent porphyria (AIP) — the most common acute porphyria — deficiency of porphobilinogen deaminase (also called hydroxymethylbilane synthase, the third enzyme in the pathway) causes accumulation of ALA and porphobilinogen (PBG), which are neurotoxic. These precursors cause autonomic neuropathy (tachycardia, hypertension, abdominal pain, vomiting, constipation), peripheral motor neuropathy (weakness, paralysis), and central nervous system dysfunction (seizures, psychosis, encephalopathy). Acute attacks are triggered by factors that upregulate hepatic ALAS1 (the...