The Clinical Diagnosis of Porphyrias by Tandem Mass Spectrometry

Abstract

Heme, a tetrapyrrole ringed molecule essential for life, is synthesized by the body in an eight-step, enzyme-assisted pathway. Each step is catalyzed by an enzyme; a genetic deficiency in any of the last seven of these enzymes is classified as a type of the family of diseases called the porphyrias. Tandem mass spectrometry, which has been previously shown by the Turecek lab to be of use in both newborn screening and in clinical diagnostics, has the potential to be used in the diagnosis of the porphyrias, as the sensitivity and selectivity of the technique can be utilized to develop more efficient and effective assays than those that currently exist. Here, work towards three new assays is described: an assay for the enzyme 5-aminolevulinic acid dehydratase, deficiency in which causes 5-aminolevulinic acid dehydratase-deficient porphyria (ADP), that utilizes erythrocytes as an enzyme source and detects a butyrated version of the enzymatic product quantified by a deuterated version of the butyrated compound; an assay for the enzyme ferrochelatase, deficiency in which causes erythropoietic protoporphyria, that utilizes mononuclear blood cells (primarily lymphocytes) as an enzyme source and detects the enzymatic product of two unnatural substrates, cobalt mesoporphyrin IX, quantifying by comparison to cobalt protoporphyrin IX; and an as-yet-incomplete assay for the enzyme uroporphyrinogen III synthase, deficiency in which causes congenital erythropoietic porphyria.