|Donini S, Ferrari M, Fedeli C, Faini M, Lamberto I, Marletta AS, Mellini L, Panini M, Percudani R, Pollegioni L, Caldinelli L, Petrucco S, Peracchi A.
|Recombinant production of eight human cytosolic aminotransferases and assessment of their potential involvement in glyoxylate metabolism
|Primary hyperoxaluria type 1 (PH1) is a severe inborn disorder of glyoxylate metabolism, caused by a functional deficiency of the peroxisomal enzyme alanine:glyoxylate aminotransferase (AGXT), which converts glyoxylate to glycine using L-alanine as the amino-group donor. Even though pre-genomic studies indicate that other human transaminases can convert glyoxylate to glycine, in PH1 patients these enzymes are apparently unable to compensate for the lack of AGXT - perhaps due to their limited levels of expression, or to their localization in an inappropriate cell compartment, or to the scarcity of the required amino-group donor. Herein, we describe the cloning of eight human cytosolic aminotransferases, their recombinant expression as His6-tagged proteins and a comparative study on their ability to transaminate glyoxylate, using any standard amino acid as an amino-group donor. To selectively quantitate the glycine formed, we have developed and validated an assay based on bacterial glycine oxidase (GO); this assay allows the detection of enzymes that produce glycine by transamination, in the presence of mixtures of potential amino-group donors and without separation of the product from the substrates. We show that among the eight enzymes tested only alanine transaminase (GPT) and phosphoserine aminotransferase (PSAT1) can transaminate glyoxylate with good efficiency, using L-glutamate (and, for GPT, also L-alanine) as the best amino-group donor. These findings confirm that glyoxylate transamination can occur in the cytosol, in direct competition with the conversion of glyoxylate to oxalate. The potential implications for the treatment of primary hyperoxaluria are discussed.