For diagnostic purposes, liver enzymes are usually classified into hepatocellular and cholestatic. These two groups of equine liver-specific enzymes include sorbitol dehydrogenase (SDH), glutamate dehydrogenase (GLDH), γ-glutamyl transferase (GGT), aspartate aminotransferase (AST), lactate dehydrogenase (LDH), and alkaline phosphatase (ALP). SDH and GLDH mostly reflect hepatocellular injury and cholestasis, while GGT expresses high values in biliary necrosis or hyperplasia. Likewise, AST, LDH, and ALP also reflect hepatocellular and biliary disease, but these enzymes are not liver specific. From the clinical point of view of the course of liver or biliary disease, AST and ALP are indicative of chronic disease, whereas SDH, GGT, and GLDH indicate an acute course.
Origin | Type | Location | Function (Catalysis) |
---|---|---|---|
Hepatocellular | SDH | Cytoplasmatic | Conversion of fructose to sorbitol |
GLDH | Mitochondrial, in the centrilobular areas of the liver | Conversion of glutamate to 2-oxoglutarate | |
AST | Cytoplasmatic of hepatocytes and other tissues, including skeletal muscle | Conversion of aspartate and alpha-ketoglutarate to oxaloacetate and glutamate | |
LDH-5 | Cytoplasmatic | Reversible transformation of pyruvate to lactate | |
Biliary | GGT | Microsomal membranes in the biliary epithelium, and also in the canalicular surfaces of hepatocytes <5% is found in the cytoplasm |
Cleaves C-terminal glutamyl groups from amino acids and transfers them to another peptide or amino acid. GGT is important in glutathione metabolism (reduced and oxidized GSH are the main targets) and amino acid absorption (cysteine in the kidney) |
ALP | Epithelium of biliary canalicular membrane, and sinusoidal membrane of hepatocytes | Non-specific metalloenzyme which hydrolyzes many types of phosphate esters at an alkaline pH in the presence of zinc and magnesium ion |
Enzyme | Reference Value Interval (UI/l) | Half-Life | Sensitivity | Specificity | Stability |
---|---|---|---|---|---|
SDH | 2–8 | <12 h | +++ | ++++ | + |
Mild | |||||
GLDH | 2–10 In foals, GLDH increase is compared to adult |
12–24 h | +++ | +++ | ++ |
AST | 150–300 | 7–8 days | +++ | + | ++++ |
GGT | 5–20 | 3 days | ++++ | +++ | ++++ |
ALP | 120–250 Foal and growing: 100-fold greater than in adults |
3 days | ++ | + | ++++ |
Increase | Number of Times Higher than the Upper Reference Interval |
---|---|
<5 times | |
Moderate | 5–10 times |
Marked | >10 times |
Number of Times Higher than the Upper Reference Interval | Pattern |
---|---|
>10-fold increase in SDH, GLDH, and/or AST, and <3-fold in GGT and/or ALP | Hepatocellular predominance (acute necrosis, ischemic or toxic damage to hepatocyte) |
>5–10-fold increase in GGT and/or ALP, and <3-fold in SDH, GLDH and/or AST | Biliary predominance (cholestasis, cholangitis, choletithus) |
>10-fold increase in SDH, GLDH and/or AST and GGT and or ALP | Mixed liver injury (acute hepatocellular and biliary damage) |