Alanine aminotransferase (ALT) is a pyridoxal enzyme Found mainly in the liver and kidney, but also in small amounts in the heart, muscle, fat, and brain. Serum aminotransferase activities have been used broadly as surrogate markers for tissue injury and disease in human and veterinary clinical settings and in safety assessment of chemicals and pharmaceuticals. Because of its relative abundance in liver, increased serum ALT activity is generally considered indicative of liver damage. Two ALT isoenzymes, ALT1 and ALT2, are known and have been cloned and sequenced from human, rat, and Mouse. In this study, we have cloned the complementary DNA encoding the canine orthologue of ALT1 (cALT1). The complete cDNA sequence comprised 1852 bases and contained a 1485-base open reading frame, which encodes a polypeptide of 494 amino acid residues. Canine ALT1 shares 87.7, 87.2, and 87.0'/o amino acid identity to its human., mouse, and rat orthologues, respectively. The cDNA was expressed in Escherichia coli. with a N-terminal His (6x) tagy, and the recombinant enzyme was purified using immobilized metal-affinity chromatography. The final yield of the purified recombinant cALT1 was greater than 5 mg/L culture. The alanine transaminase activity Of purified cALT1 was 229.81 U/mg protein, which is approximately 38-fold higher than that of total soluble recombinant E. coli cell lysate, confirming that the enzyme is a functional ALT. Evaluation of various canine tissues by RT-PCR revealed that the level of ALT1 expression is in the order of: heart > liver > fat similar to brain similar to gastrocnernius > kidney. The purified cALT1 will be helpful to develop isoenzyme-specific anti-bodies. which could further improve the diagnostic resolution of current ALT assays in drug safety studies. (c) 2006 Elsevier Inc. All rights reserved.