Iditol dehydrogenase (ID)
Sorbitol dehydrogenase (SDH) is found in the highest concentration in the liver. It is a cytoplasmic enzyme that catalyzes the conversion of fructose to sorbitol. It is a very specific indicator of liver disease in all species, although increases can occur with primary or secondary liver disease (e.g. many horses with inflammatory gastrointestinal disease will have high SDH activity). Increases in activity occur within 24 hours of liver injury. SDH is the enzyme of choice for detecting hepatocellular injury in horse and cattle (as AST and LDH are not liver-specific), particularly in fresh samples, and is a component of the current Large Animal Chemistry Profile offered by the clinical pathology laboratory at Cornell.
The half life is reported to be 5 hours in the dog, 3-4 hours in the cat and 12-24 hours in the horse.
SDH catalyses the conversion of D-fructose to D-sorbitol. The rate of oxidation of NADH is directly proportional to the rate of conversion of D-Fructose to D-Sorbitol. The declining rate of NADH is measured photometrically and is proportional to SDH activity.
D-fructose + NADH ↔ SDH > D-sorbitol + NAD+
Units of measurement
SDH activity is measured in U/L (international units).
SDH is not a stable enzyme. Due to this instability, GLDH (which is reportedly more stable than SDH) is also included on our large animal chemistry panels.
- Cattle and horses: Serum – 5 hours at room temperature, 24 hours at 4°C and several days frozen (Horney et al 1993). Note, in the latter study, all tested animals had SDH activity within reference intervals.
- Llamas: Stability is indicated below for separated samples. There was no difference in baseline activity comparing separated serum to heparinized plasma (Tornquist et al 2000).
- Serum: Stable for <24 hours room temperature (decreased by a mean of 21% versus baseline), 1 week refrigerated or frozen at -20ºC.
- Plasma (heparin): Stable for 24 hours room temperature (decreased by a mean of 6%), 1 week refrigerated or frozen at -20ºC (Tornquist et al 2ooo). Note the latter study was done in 11 llamas and all had SDH activity within reference intervals.
- Lipemia, hemolysis, and icterus: Hemolysis and lipemia may increase results per product information sheet. However, internal studies with equine serum spiked with equine RBC hemolysate showed that SDH activity was decreased from 5 U/L to undetectable at a hemolytic index of 492 units (marked hemolysis) with the Hitachi P modular analyzer.
Increased activity is an indicator of hepatocellular damage, which may be reversible or irreversible.
- Artifact: Delayed analysis may decrease results due to enzyme instability (Horney et al 1993). We have observed falsely increased activity in some mailed-in samples. Degradation of the enzyme into component subunits may be responsible for this artifact.
- Hepatic injury: Indicates hepatocyte damage, but is not specific for the type of injury. In a carbon tetrachloride model of hepatic injury in 7 horses, mean SDH activity was increased above baseline at 4 hours and peaked at 2 days after administration. Results for AST activity lagged slightly behind SDH. SDH activity returned to baseline by 5 days (and were markedly decreased by 4 days) after administration (Bernard and Divers 1989). Similar results were reported for a study of chloroform-induced hepatocellular injury in 3 horses (Barakat and Ford 1988). In the latter study, SDH and GLDH peaked at 2 days in 1 of two sheep after administration of chloroform and declined to baseline values by 5-6 days, whereas activity of both enzymes peaked at 1 day in 4 calves, with a return to baseline values within 9-10 days (no standard deviations provided). In horses with experimental equine parvovirus infection, SDH activity increases just after the peak of viremia, with increases in several horses matching that seen in GLDH activity (although the latter did increase earlier and stayed increased longer than SDH in a few cases (Tomlinson et al 2020)
This is not of clinical relevance.