The investigation of inborn errors in vivo using stable isotopes.

By labelling compounds with stable isotopes metabolic events can be studied in vivo. Pathways can be investigated and both enzyme and substrate kinetics measured. The concept of labelling substrates to investigate metabolic events in vivo is straightforward but there are potential sources of error that must be recognised. Patients with a wide range of inborn errors have now been studied. In some there is negligible residual enzyme activity but in others, even those with no activity in vitro, residual enzyme activity is apparently present in vivo, probably as a result of metabolic activity of alternative pathways. These studies have demonstrated the importance of such pathways and have provided insight into the biochemical adaptations that occur in patients with inherited metabolic disorders.