Alanine aminotransferase (ALT) is a widely used index of liver integrity or hepatocellular damage in clinics as well as a key enzyme in intermediatary metabolism. In this study, we have cloned the complementary DNAs of murine homologues of human alanine aminotransferase 1 and 2 (ALT1 and ALT2). The deduced peptides of murine ALT1 (mALT1) and ALT2 (mALT2) share 87% and 93% identity, respectively, with their human counterparts at the amino acid level. Murine ALT genes localize to separate chromosomes, with mALT1 gene (gpt1) on chromosome 15 and mALT2 gene (gpt2) on chromosome 8. The murine gpt1 and gpt2 differ in messenger RNA expression: gpt1 is mainly expressed in liver, bowel, and white adipose tissue and gpt2 is highly expressed in muscle, liver, and white adipose tissue. Expression of recombinant mALT1 and mALT2 proteins in Escherichia coli (E. coli) produced functional enzymes that catalyze alanine transamination. The potential diagnostic value of ALT isoenzymes in liver disease was evaluated in an obese animal model. In fatty livers of obese mice, ALT2 gene expression is induced 2-fold, but ALT1 remains the same. Furthermore, in fatty liver, total hepatic ALT activity is elevated significantly by 30% whereas aspartate aminotransferase (AST) activity remains unchanged. In conclusion, these results indicate that ALT2 may be responsible for the increased ALT activity in hepatic steatosis and provide evidence that an ALT isoenzyme-specific assay may have more diagnostic value than the total ALT activity assay currently in clinical use.