The human hydroxyacylglutathione hydrolase (HAGH) gene encodes both cytosolic and mitochondrial forms of glyoxalase II

In yeast and higher plants, separate genes encode the cytosolic and mitochondrial forms of glyoxalase II. In contrast, although glyoxalase II activity has been detected both in the cytosol and mitochondria of mammals, only a single gene encoding glyoxalase II has been identified. Previously it was thought that this gene ( the hydroxyacylglutathione hydrolase gene), comprised 8 exons that are transcribed into mRNA and that the resulting mRNA species encoded a single cytosolic form of glyoxalase II. Here we show that this gene gives rise to two distinct mRNA species transcribed from 9 and 10 exons, respectively. The 9-exon-derived transcript encodes two protein species: mitochondrially targeted glyoxylase II, which is initiated from an AUG codon in a previously uncharacterized part of the mRNA sequence, and cytosolic glyoxalase II, which is initiated by internal ribosome entry at a downstream AUG codon. The transcript deriving from 10 exons has an in-frame termination codon between the two initiating AUG codons and hence only encodes the cytosolic form of the protein. Confocal fluorescence microscopy indicates that the mitochondrially targeted form of glyoxalase II is directed to the mitochondrial matrix. Analysis of glyoxalase II mRNA sequences from a number of species indicates that dual initiation from alternative AUG codons is conserved throughout vertebrates.