Enzymes act within the cells (exception: anti- and procoagulatory enzymes). Increased activities are detected when cell membrane permeability is disturbed and enzymes leak into the extracellular fluid. Increased enzyme activities do not necessarily indicate a decreased organ function. Enzyme profiles localize the injuries (e.g. muscle or liver), sequential activity measurements help judging the time course.

ALP, Alkaline Phosphatase

The enzyme is localized in cell membranes of many organs as liver, kidney, intestines, mammary gland and bone;

Interpreation: young animals show increased activities due to sceletal growth, in dogs and horses AP is a sensitive marker of cholestasis. Drug therapy can increase alkaline phosphatase activities in dogs (e.g. corticosteroids, anticonvulsants). Fatty liver disease in cats is characterized by up to ten fold increases of alkaline phosphatase activities with only moderately increased GGT elevations .

c-ALP, steroid induced alkaline Phosphatase

Corticosteroids induce the elevation of a hypoglycosilated form of AP. 95% of dogs with hyperadrenocorticism show marked elevations of steroid induced AP activity. 

Indication: suspect hyperadrenocorticism, elevated AP activitiy

ALT – Alanin transferase 

is localized within the cytosol of liver and muscle cells;

In dogs, cats, primates, rabbits and rats it is specific for liver damage. Elevations indicate increased permeability of liver cell membranes, that peak within 12 hours after injury; The magnitude of the increase is correlated with the number of injured cells but etiology, severity and an eventual loss of liver function cannot be deduced.

AST – Aspartate transferase

is localized within the cytosol and mitochondria of liver and muscle cells.

In cats, cattle and pigs high specifity for liver damage. Increases of the mitochondrial fraction indicates cellular necrosis, but the differentiation of the two fractions is methodically difficult and not for routine use. 
Parallel increases of AST and CK indicate soft tissue damage.

GLDH – Glutamate Dehydrogenase

Localized with the mitochondria. High activity within centrilobular liver cells.

High specifity for liver injuries in all species. Hypoperfusion of liver tissue induces solitary increases, parallel increases with transaminases indicate liver cell necrosis.

GGT, Gamma-glutamyl transferase

Is a cytosolic an membrane-bound enzyme found in highest concentrations in the brush borders of renal and bile duct epithelium. 

Veryl low basal activities in cats and rats; Marker for cholestasis. Induction due to corticosteroids and anticonvulsants in dogs. 
High activity in newborns after intake of colostrum.

CHE – Cholinesterase

Indication: suspected organophosphate-toxicosis (decreased activitiies).

Amylase

Localized within pancreas, salivary glands, intestines and liver. Activity is highest within pancreas.

Indication: clinical signs suggest acute pancreatitis e.g. vomiting, abdominal pain. Rapid increase after beginning pancreatitis, rapid decrease. 
Non specific elevations in bowel disease and reduced glomerular filtration rate (only slightly elevated).

Lipase

Localized within pancreas and stomach.

Indikation: clinical signs associated with pancreatitis e.g. vomiting, abdominal pain. Non specific elevations in small intestine disease.

CK, Creatinkinase

Localized within muscle cells.

Indication: soft tissue damage, differentiation of elevated ALT and AST activities.

Highly sensitive, increased with minor blunt trauma. Increase within the first 24 hours, nomalized within 8 days after cessation of injury.
Non specific indicator of central nervous disease – elevated activities in CSF. 
Determination of isoenzymes has no relevance in veterinary medicine.

LDH – Lactate dehydrogenase

Non specific increases with inflammatory and degenerative processes. Extreme elevations in body fluids might indicate a tumor.