Why is the patient with acute intermittent porphyria not light sensitive?

Acute intermittent porphyria (AIP) belongs to the group "inborn errors of metabolism". The disease is inherited as an autosomal dominant gene and characterized by abdominal, neurological and neuropsychiatric symptoms. The basic defect in AIP is assumed to be a mutation in the genetic system which controls the uroporphyrinogen-I-synthetase (UIS) activity. The UIS activity in erythrocytes is about 50% lower in gene carriers than among their non-porphyric siblings. The metabolic block in the haem synthesis is situated between porphobilinogen and uroporphyrinogen and leads to high levels of porphyrin precursors but no accumulation of porphyrins. Since the porphyrin precursors do not cause light sensitivity and tissue porphyrins are not markedly increased most patients with AIP are not light sensitive.