The early effects of short-term dexamethasone administration on hepatic and serum alanine aminotransferase in the rat.

Dexamethasone (DEXA) administration has been associated with serum alanine aminotransferase (ALT) elevations that may result from enhanced ALT expression. The aim of our current study was to compare liver vs. serum ALT activity and to examine the onset of any hepatocellular changes. Groups of 4 male Sprague-Dawley rats were administered a single dose of DEXA or corn oil at 12, 16, and 24 h prior to euthanasia or once-daily for 2, 3, or 4 days. All (nonfasted) rats were necropsied together on Day 5. While DEXA incrementally increased liver ALT activity in the 1-, 2-, 3-, and 4-day treatment groups (maximal, 3.7-fold), liver aspartate aminotransferase (AST) never exceeded 1.4-fold over control. Significant hepatic glycogen elevations were detected after DEXA treatment, which correlated with microscopic observations. Serum ALT, AST, sorbitol dehydrogenase, and glutamate dehydrogenase (GLDH) increased after 2, 3, and 4 days of DEXA dosing (1.3-10.3-fold). DEXA-related necropsy findings included pale livers consistent with glycogen deposition. The relative percent liver to body weight was elevated in all DEXA-treated rats. Hepatocellular necrosis was observed in 1/4 rats at 12 h, 2/4 rats at 2 days, 4/4 rats at 3 days, and 3/4 rats at 4 days. DEXA treatment <2 days failed to produce consistent evidence of hepatic injury, as detected by serum biomarkers and pathology assessment. However, early DEXA treatment did correlate with apparent ALT induction. Ultimately, this may explain some early asymptomatic serum ALT elevations seen clinically.

cDNA cloning, expression, purification, distribution, and characterization of biologically active canine alanine aminotransferase-1.

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% amino acid identity to its human, mouse, and rat orthologues, respectively. The cDNA was expressed in Escherichia coli, with a N-terminal His (6x) tag, and the recombinant enzyme was purified using immobilized metal-affinity chromatography. The final yield of the purified recombinant cALT1 was greater than 5mg/L culture. The alanine transaminase activity of purified cALT1 was 229.81U/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 approximately brain approximately gastrocnemius>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.