Hydroxymethylbilane synthetase (HMBS) converts porphobilinogen to hydroxymethylbilane. A defect in this enzyme results in acute intermittent porphyria (AIP)


Synonyms for HMBS: porphobilinogen deaminase, PBG deaminase, uroporphyrinogen I synthase


Inheritance: autosomal dominant


Genetic locus: 11q23.3


Clinical features:

(1) neurovisceral signs and symptoms

(2) without skin lesions

(3) family history


Laboratory findings (more pronounced during an acute attack):

(1) elevated urine porphobilinogen, aminolevulinic acid and uroporphyrins

(2) normal to slightly elevated plasma porphyrins

(3) usually no elevation of porphyrins in the feces but an excess may occur (coproporphyrinogen, protoporphyrin)

(4) no increase in protoporphyrins within erythocytes

(5) decreased erythrocyte porphobilinogen deaminase levels


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