Alanine glyoxylate aminotransferase deficiency: biochemical and molecular genetic lessons from the study of a human disease.

The decision to treat a patient with primary hyperoxaluria type 1 (PHI) by combined liver and kidney transplantation, the former to correct the metabolic lesion which was then thought to be deficiency of cytoplasmic 2-oxoglutarate:glyoxylate carboligase, and the latter to replace the organ which is destroyed, provided an opportunity to investigate the disease by modern biochemical methods. It was shown that 2-oxoglutarate:glyoxylate carboligase (the first decarboxylating component of 2-oxoglutarate dehydrogenase) is entirely mitochondrial so that deficiency of a cytoplasmic form of this enzyme could not be the cause of PHI. The deficient enzyme proved to be hepatic peroxisomal alanine:glyoxylate aminotransferase (AGT). The disease can be diagnosed enzymologically on percutaneous liver biopsies and this is possible for the fetus in utero. There are four types of genetically determined heterogeneity in PHI:(1) responsiveness and non-responsiveness to pharmacological doses of pyridoxine, in terms of an effect on the rate of oxalate production; (2) the presence or absence of residual catalytic AGT activity; (3) CRM+ and CRM-variants; (4) locational variation by virtue of which the enzyme (AGT) is mitochondrial and not peroxisomal. About one third of patients with PHI have residual AGT activity and at least a large proportion of these have mitochondrial and not peroxisomal AGT. The molecular features which guide peroxisomal and mitochondrial enzymes from their sites of synthesis into the appropriate organelle are reviewed and the possibilities for genetic variation in the relevant parts of the AGT molecule are discussed. The gene directing the synthesis of AGT has been cloned and sequenced, as has the AGT cDNA from a patient with mitochondrial AGT. Three point mutations causing amino acid substitution in the predicted AGT protein sequence have been identified: proline----leucine at residue 11, glycine----arginine at residue 170 and isoleucine----methionine at residue 340. The present evidence based on screening PHI patients and control subjects suggest that the substitution at residue 11, which cosegregates with that at residue 340, generates an amphiphilic alpha-helix which resembles mitochondrial targeting sequences but that misrouting of all the newly synthesized AGT into mitochondria requires the substitution at residue 170 which may act by impeding the entry of the enzyme into peroxisomes. The recognition of enzyme locational heterogeneity in PHI due to mutations affecting leader sequences should encourage a search for similar metabolic lesions in other inborn errors of metabolism affecting peroxisomal and/or mitochondrial enzymes.