Primary hyperoxaluria type 1 and peroxisome-to-mitochondrion mistargeting of alanine:glyoxylate aminotransferase

Abstract

Under the influence of dietary selection pressure, the intracellular compartmentalization of alanine:glyoxylate aminotransferase (AGT) has changed on many occasions during the evolution of mammals. In some mammals, AGT is peroxisomal in others it is mainly mitochondrial while in yet others it is more-or-less equally divided between both organelles. Although in normal human liver AGT is usually found exclusively within the peroxisomes, in some individuals a small proportion (approximately 5%) is found also in the mitochondria. This apparently trivial intracellular redistribution of AGT is caused by the presence of a Pro11Leu polymorphism which allows the N-terminus of AGT to fold into a conformation (ie a positively-charged amphiphilic alpha-helix) which functions as a mitochondrial targeting sequence. In one third of patients with the autosomal recessive disease primary hyperoxaluria type 1, there is a further redistribution of AGT so that the great majority (approximately 90%) is located in the mitochondria and only a small minority (10%) in the peroxisomes. AGT cannot fulfil its proper metabolic role in human liver (ie glyoxylate detoxification) when located in the mitochondria. The erroneous compartmentalization is due to the presence of a Gly170Arg mutation superimposed upon the Pro11Leu polymorphism. The Gly170Arg mutation appears to have no direct effect on mitochondrial targeting and is predicted to enhance mitochondrial import of AGT by interfering with its peroxisomal targeting and/or import. The mitochondrial targeting sequence generated by the Pro11Leu polymorphism is not homologous to that found in the AGT of other mammals which localise AGT within the mitochondria normally. The identity of the peroxisomal targeting sequence in AGT is unknown, but the Gly170Arg mutation is found in a highly conserved region of the protein which might be involved in some aspects of the peroxisomal import pathway for AGT.