Deuterium isotope effects on porphobilinogen synthesis catalysed by 5-aminolaevulinic acid dehydratase

Deuteriation of 5-aminolaevulinic acid (ALA) at C-5 has no effect on the rate of porphobilinogen synthesis by ALA dehydratase from Bacillus subtilis but deuteriation at C-3 gave isotope effects on kcat and kcat/KM of 3.4 and 2.3 respectively. Reisolated ALA after 50% reaction shows no significant loss of deuterium at C-3, indicating that it is probably the first deprotonation at this carbon which is rate-determining.