Lactate dehydrogenase (LDH, LD) is an enzyme that catalyzes the conversion of lactate to pyruvate. It is not tissue-specific, being found in a variety of tissues, including liver, heart and skeletal muscle. The enzyme is tetrameric and is composed of four subunits of two molecules, M (muscle) and H (heart). Various combinations of these two molecules result in five different isoenzymes.
- LDH1: This is composed of four H subunits (H4) and is found mostly in cardiac muscle and erythrocytes in small animals, whereas this isoenzyme is found mostly in the kidney and liver of sheep and cattle.
- LDH2: This is composed of three H subunits and one M subunit (MH3). LDH2, LDH3 and LDH4 are found in all tissues.
- LDH3: This is composed of two H and two M subunits (M2H2).
- LDH4: This is composed of one H and three M subunits (M3H).
- LDH5: This is composed of four M subunits (M4) and is found in skeletal muscle in all species and the liver in horses and small animals.
Method of measurement
Total LDH can be measured in serum,plasma or body cavity fluids such as cerebrospinal fluid (CSF). The method given below is the one used at Cornell University to measure total LDH. LDH isoenzymes can be semi-quantified using agarose electrophoresis (Houle et al 2015).
UV kinetic assay
LDH catalyzes the conversion of L-lactate to pyruvate. In the process NAD is reduced to NADH which results in the production of L-glutamate and oxaloacetate. This reaction is carried out to equilibrium. Oxaloacetate produced in the first reaction is then reduced by NADH to L-malate under the catalytic action of malate dehydrogenase (MDH). The increasing rate of NADH is measured photometrically and is proportional to LDH activity.
L-lactate + NAD+ < LDH > pyruvate +NADH + H+
Units of measurement
The activity of lactate dehydrogenase is expressed in U/L (International units) and µkat/L (SI units), which is defined as the amount of enzyme that catalyzes the conversion of 1 µmol of substrate per minute under specified conditions. Conversion formula is shown below:
U/L x 0.0167 = µkat/L
Serum, plasma, CSF
The stability of LDH in human serum and plasma samples per the manufacturer reagent product information sheet is as follows: 7 days at 15-25 °C, 4 days at 2 – 8 °C, and 6 weeks at (-15)-(-25) °C.
- Serum: Total LDH is stable for 4 weeks at 4ºC or -20ºC.
- CSF: Loses around 30% activity within 24 hours at 4ºC, with greater retention of activity (10-15% loss) if stored at -20ºC for up to 3 weeks (Houle et al 2015).
- Lipemia: Severe lipemia (>1000-1500 lipemia index) may decrease concentrations.
- Hemolysis: Will increase activity due to release of intra-erythrocyte LDH (> 15 hemolytic index in humans). In pigs, even mild hemolysis will increase results (median 69 U/L) (di Martino et al 2015). We have observed that LDH concentrations are directly correlated to the hemolytic index in dogs, therefore high concentrations in a hemolyzed sample may be entirely as a consequence of hemolysis.
- Icterus: Only high concentrations of bilirubin (>60 units on icteric index) will affect results.
Increased LDH activity
- Artifact: Hemolysis (LDH is high in erythrocytes in dogs, cats and pigs); serum has higher activity than plasma as the enzyme is released from cells during clotting. In our experience, this is a common artifact in dogs.
- Physiologic: During exercise LDH activity rise to meet the increased generation of lactic acid.
- Liver injury: Hepatocellular injury will increase LDH1 and LDH2 in cattle and sheep and LDH5 in horses and small animals. In one study of 7 horses given carbon tetrachloride, LDH5 activity increased above baseline within 4 hours of administration and stayed increased to peak at 2 days. Activity then declined to within reference intervals by days 4-5. Changes in LDH5 paralleled but were proportionally lower in magnitude changes in SDH activity (Bernard and Divers 1989). Some cats with liver injury have high LDH activity (when CK activity is normal and the sample is not hemolyzed).
- Muscle disease: This will increase LDH5 in sheep, cattle and horses, e.g. selenium and vitamin E deficient myopathy in cattle and sheep, exertional rhabdomyolysis in horses. We have seen high LDH activity in cats with high CK activity, suggesting that LDH does increase with muscle injury in cats.
- Neoplasia: LDH is increased in various malignancies in dogs and cattle (e.g. lymphoma). However, high activity is also found in dogs without cancer, raising questions as to the diagnostic usefulness of LDH as a screening tool for occult cancer (Marconato et al 2009).
Because LDH is so non-specific and isoenzyme measurement is not routinely available, its measurement does not confer any additional information about skeletal muscle or hepatic disease in domestic animals than that provided by enzyme assays routinely used for this purpose (i.e. CK for muscle and SDH/GLDH or ALT for liver in large and small animals, respectively).