Physiology
Glutamate dehydrogenase (GLDH, GMD, GLD, and GDH, although GMD is preferred by one group of authors [Baron et al 1975]) is a mitochrondrial enzyme that catalyzes the conversion of glutamate to 2-oxoglutarate. Increases in GLDH activity are used primarily to reflect leakage from damaged or necrotic hepatocytes. Since it is quite a large mitochondrial enzyme, injury needs to be sufficiently severe to damage mitochondria. GLDH is a useful enzyme for hepatocellular injury in large animals and exotic species (birds, amphibians, reptiles). Low activity of GLDH (usually < 10 U/L) are seen in health in small animals, horses and donkeys, whereas healthy cattle and alpacas may have higher activity (up to 60 U/L in cattle and sheep, and 20 U/L in alpacas) based on our reference intervals.
The half life of GLDH is about 8 hours in the dog (Zinkl et al 1971, n=1 after injection of liver homogenate) and 14 hours in cattle (Collis et al 1979).
Organ specificity
GLDH is found in many tissues in the body, including hepatocytes, kidney, intestine, muscle, and salivary gland. However, most of serum GLDH originates from hepatocytes (in health and disease states). In horses, GLDH activity is highest in the liver, with only small amounts being seen in heart, jejunum and skeletal muscle (Johnson et al 2019). GLDH is located more in the centrilobular areas of the liver (Wimmer and Pette 1979), whereas AST is more homogenously distributed and ALT is more periportal in rats. Due to its preferential location in centrilobular areas, liver injury involving these areas (e.g. hypoxia) may result in higher activity of GLDH than ALT.
Method
The following photometric method is used at Cornell University.
Reaction type
Kinetic
Procedure
As NADH is oxidised, the rate of decrease in absorbance is measured photometrically and is proportional to the GLDH activity. The reaction is shown below.
α-oxoglutarate + NADH + NH4+ GLDH > glutamate + NAD+ + H2O
Units of measurement
GLDH activity is reported in U/L. One unit of GLDH is the amount of enzyme that will generate 1 μmol of NADH per minute at pH 7.6 at 37 °C.
Sample considerations
Sample type
Serum, plasma
Anticoagulant
Heparin. GLDH is a zinc-dependent enzyme and EDTA will lower its activity by chelating zinc (Hoffman).
Stability
GLDH is a component of the current Large Animal and Non-mammalian Chemistry Profile offered by the clinical pathology laboratory at Cornell. This is because it is thought to be a more stable enzyme (with storage) than SDH (which decreases rapidly in stored samples).
Interferences
- Lipemia, hemolysis, and icterus: The effects of these interferents on GLDH are currently unknown. A small internal study using equine serum spiked with equine red blood cell hemolysate at Cornell University showed that GLDH activity increased with a hemolytic index >1000 (marked) on the Hitachi P modular (by 120% at a hemolytic index of 1282 units and by 220% at a hemolytic index of 3568 units).
Test interpretation
Increased activity
- Physiologic: GLDH activity in some foals may be above reference intervals established for adults (similar to GGT) and may not indicate liver injury (unpublished observations).
- Pathophysiologic:
- Liver injury: GLDH is a sensitive and specific marker of liver disease in all animals, including non-mammalian species. In rats, increases in GLDH were greater in magnitude, persisted longer or occurred without concurrent increases in ALT in drug-induced hepatic injury. In calves, horses and sheep, changes in GLDH paralleled changes in SDH after acute injury induced by chloroform. Activity generally peaked at 1-2 days and then decreased to reference intervals within 4-9 days (Barakat and Ford 1988). Our experience with horses at Cornell University is that GLDH is usually increased concurrently with SDH in liver damage, however activities often remain above reference intervals for longer (therefore, it may be a more sensitive marker of persistent liver injury than SDH) and the magnitude of increase is greater than with SDH. The latter was confirmed in a study of horses with experimental equine parvovirus infection. Increases in GLDH activity preceded (2-7 days) and was of greater magnitude (versus the upper reference limit) than increases in SDH activity and in a few horses, the GLDH activity was still high for 2-10 days after SDH activity had normalized (Tomlinson et al 2020). GLDH increased to up over 600 U/L in one horse, with most horses having increases <200 U/L. The increased liver enzyme activities was seen between 6-10 weeks after infection. Increases in both enzymes were seen just after the peak viremia and there was evidence of hepatocellular necrosis in parvovirus-infected hepatocytes (using in situ hybridization to detect the virus). The increase in GLDH activity also preceded the increase in AST activity. The higher apparent sensitivity of GLDH versus SDH for parvovirus hepatitis can be explained by the inflammation identified in the liver, which was mostly within the centrilobular region (Tomlinson et al 2020), which contains the highest concentration of GLDH in rat livers. The inflammation in the experimentally infected horse livers was comprised of a mixture of neutrophils, CD3+ T cells and macrophages (Iba-1 positive) (Tomlinson et al 2020). GLDH activity is not increased in horses or exotic animals with severe muscle injury (extremely high CK activity), supporting its specificity for liver injury. Like SDH, GLDH is not specific as to the cause of liver injury.
Decreased activity
This is not of clinical relevance.
References
- Hoffman WE and Solter PF. Diagnostic enzymology of domestic animals. In: Clinical Biochemistry of Domestic Animals, eds. Kaneko JJ, Harvey JW and Bruss M. 6th edition. Academic Press. 2008; pp:351-378. https://doi.org/10.1016/B978-0-12-370491-7.X0001-3
