5.2 Clinical Chemistry: Biomarkers of Hepatic, Renal & Muscle Injury
Key Takeaways
- ALT, AST, SDH, and GLDH are leakage enzymes of hepatocellular (and, for AST, muscle or erythrocyte) injury; ALP and GGT more often rise by induction in cholestasis or after certain enzyme-inducing treatments.
- Centrilobular necrosis typically lights up ALT/AST/GLDH/SDH first; biliary hyperplasia and cholestasis typically light up ALP, GGT, and bilirubin—do not treat “liver enzymes” as one interchangeable pool.
- BUN and creatinine are late, imperfect GFR markers; SDMA and cystatin C can detect GFR decline with less muscle-mass dependence but remain adjuncts to urinalysis, kidney weights, and histopathology.
- CK is the muscle leakage enzyme of choice; pair it with AST and ALT so injection-site or restraint injury is not reported as hepatotoxicity.
- Fasting, in-vitro hemolysis, and age (especially bone ALP in juveniles) routinely mimic or mask toxicant effects; interpret chemistry against concurrent, similarly handled controls.
Why chemistry is more than starred means
Domain I.C.2 continues from the CBC into clinical chemistry: enzymes, metabolites, electrolytes, proteins, and lipids that report organ injury or function while the animal is still alive. Independent OpenExamPrep material here trains you to name the pattern (hepatocellular leakage, cholestasis, decreased GFR, muscle injury, artifact) before you name a target organ. A table of “significantly increased liver enzymes” that never says which enzymes, and never mentions creatine kinase, histopathology, or sample quality, is not an interpretation.
This section is original teaching. It does not reproduce American Board of Toxicology sample items. The skill is the same one you need on novel stems: match the analyte family to the biology, then confirm or refute the story with concurrent controls and morphology.
Leakage versus induction enzymes
Leakage enzymes sit in cytosol or mitochondria and spill when membranes fail. The hepatocellular set is alanine aminotransferase (ALT), aspartate aminotransferase (AST), sorbitol dehydrogenase (SDH) (iditol dehydrogenase), and glutamate dehydrogenase (GLDH). Creatine kinase (CK) is the muscle counterpart. Magnitude roughly tracks the amount of injured tissue and the enzyme’s plasma half-life. A large ALT spike with necrosis can still be fully reversible if surviving hepatocytes regenerate and the insult stops.
Induction enzymes increase because the cell synthesizes more protein. Classically alkaline phosphatase (ALP) and gamma-glutamyltransferase (GGT) rise in cholestasis or after certain microsomal enzyme-inducing treatments. ALP also comes from bone (young growing animals) and, in dogs, a corticosteroid-induced isoform. High ALP in a six-week-old puppy or a rapidly growing rat is not automatically a biliary toxicant. Treating ALP as “the liver leakage enzyme” is the category error this section exists to prevent.
Half-life matters operationally. SDH is labile—delays between draw and assay lose activity. GLDH is more stable and is widely used in rodents as a relatively specific mitochondrial hepatocellular marker. AST is in liver, muscle, and erythrocytes, so hemolyzed tubes and muscle trauma both inflate it. Use AST with ALT and CK, not as a solo “liver test.”
Hepatocellular injury: ALT, AST, SDH, GLDH
ALT is the workhorse hepatocellular leakage enzyme in dog, cat, rat, and primate. Skeletal muscle contains some ALT in several species, so a muscle-injury pattern can lift ALT, but in the usual laboratory species a large ALT rise with a quiet CK still points at liver.
AST is less tissue-specific. SDH is more liver-specific in several species and is valuable when ALT is uninformative, provided the laboratory assays it promptly. GLDH reports mitochondrial hepatocellular injury and pairs well with histologic centrilobular necrosis—zone 3 is cytochrome-rich and relatively hypoxic, so many bioactivated hepatotoxicants (classic teaching examples include carbon tetrachloride-type injury) declare themselves there.
Do not rank “which single enzyme proves hepatotoxicity” as a parlor game. The coherent package is which leakage enzymes moved, whether CK moved, whether bilirubin and ALP moved, and what the liver looks like.
Cholestasis: ALP, GGT, bilirubin
ALP, GGT, and total/conjugated bilirubin describe cholestasis and biliary induction more than pure leakage necrosis. Biliary hyperplasia, bile-duct injury, canalicular obstruction, and some enzyme inducers raise ALP/GGT. Bilirubin rises when conjugation or excretion fails or when hemolysis overloads hepatic uptake; fractionate when you can (conjugated versus unconjugated) to separate excretion failure from hemolysis.
A chemical that causes centrilobular necrosis typically lights up ALT/AST/GLDH/SDH first; bilirubin and ALP may stay modest until injury is extensive. A chemical that causes bile-duct hyperplasia or cholestasis lights up ALP/GGT/bilirubin, with ALT milder unless hepatocytes are also dying. Those two patterns are not interchangeable “liver enzyme” events.
Kidney: BUN and creatinine versus SDMA and cystatin C
Blood urea nitrogen (BUN) (urea) rises when glomerular filtration rate (GFR) falls, but also with high protein catabolism, gastrointestinal hemorrhage (absorbed nitrogen), dehydration, and high-protein diets. Creatinine tracks GFR more cleanly but depends on muscle mass; cachectic animals under-generate creatinine, masking GFR loss. Prerenal azotemia (dehydration, shock) raises BUN ± creatinine with concentrated urine and often no primary renal histologic lesion.
Symmetric dimethylarginine (SDMA) and cystatin C are low-molecular-weight markers that can detect GFR decline with less muscle-mass dependence than creatinine. In regulatory packages they are still adjuncts. BUN, creatinine, urinalysis, kidney weights, and histopathology remain the core. Do not discard a creatinine increase because SDMA was not measured, and do not call SDMA a histopathology substitute. Section 5.3 continues this story with urinary KIM-1, NGAL, and clusterin, which report tubular injury rather than GFR.
Muscle, electrolytes, glucose, lipids, proteins
CK is the muscle leakage enzyme of choice. AST rises with muscle too. Injection-site injury, restraint in NHP, intramuscular dosing, and some convulsive test articles raise CK. Pair CK with AST/ALT to avoid calling muscle leakage hepatotoxicity. Cardiac muscle injury is better supported later with cardiac troponins (section 5.3) plus heart weights and histology; CK alone does not localize heart versus skeletal muscle.
Electrolytes (sodium, potassium, chloride, bicarbonate) catch gastrointestinal loss, renal tubular dysfunction, mineralocorticoid effects, and artifact. Hemolysis raises potassium in several species. Glucose: fasting lowers it; stress and glucocorticoids raise it. Lipids: fasting, diet, cholestasis (cholesterol), and PPAR-type pharmacology. Proteins: albumin falls with hepatic synthetic failure, gastrointestinal or renal loss, or inflammation (negative acute-phase); globulins rise with inflammation. The albumin-to-globulin (A/G) ratio helps classify those patterns.
Confounders: fasting, hemolysis, age
Fasting (often overnight before a terminal bleed) drops glucose, can drop triglycerides, and may slightly change bilirubin and ALP. Compare fasted to fasted. Mixing unfasted interim bleeds with fasted terminal bleeds manufactures chemistry “findings.”
Hemolysis in the tube raises potassium, AST, lactate dehydrogenase, phosphate, and can interfere optically with many assays. Mark hemolyzed samples. Do not invent a hepatocellular AST story from a red serum when ALT and GLDH are quiet and CK is unhelpful.
Age: bone ALP is high in juveniles; young rats have different protein and enzyme baselines; geriatric rats add chronic progressive nephropathy and background liver change. Always use concurrent age-matched controls. Historical ranges help you sanity-check; they do not replace the concurrent control group.
Pattern recognition: centrilobular necrosis versus biliary hyperplasia
| Clinicopathologic pattern | Typical chemistry | Histopathology you expect to confirm |
|---|---|---|
| Acute hepatocellular leakage | ALT, AST, SDH, and/or GLDH increased; ALP/bilirubin normal or lagging | Centrilobular (or other zonal) degeneration/necrosis |
| Cholestatic / biliary induction | ALP and/or GGT increased; bilirubin increased when excretion fails | Bile-duct hyperplasia, cholestasis, periportal inflammation or fibrosis |
| Mixed hepatocellular and biliary | Both leakage and induction enzymes up | Hepatocellular injury plus a biliary reaction |
| Muscle | CK dominant; AST up; ALT milder | Skeletal or cardiac muscle; injection site |
| Decreased GFR | BUN ± creatinine up; possible hyperphosphatemia | Glomerular/tubular/interstitial change—or prerenal (no primary renal lesion) |
Scenario
High-dose dogs have ALT eight times concurrent controls, GLDH up, ALP 1.2 times control, bilirubin unchanged, CK unchanged. Histology shows centrilobular hepatocellular necrosis. That is hepatocellular leakage, not a primary cholestatic story. Writing “liver function tests are up” without naming the pattern would miss the point of I.C.2.
A different cohort has ALP six times control, GGT up, bilirubin up, ALT 1.5 times control, and histology showing bile-duct hyperplasia without necrosis. That is a biliary/cholestatic pattern. Treating the two cohorts as interchangeable because both involve the liver is the error.
A third cohort, NHP this time, has AST and CK high, ALT barely moved, and no hepatic necrosis. The chemistry is muscle (restraint or injection), not a silent hepatotoxicant.
Traps
- Calling ALP a hepatocellular leakage enzyme.
- Ignoring CK when AST is high.
- Using unfasted historical ranges on fasted terminal bleeds.
- Equating any BUN rise with tubular necrosis.
- Reading hemolyzed samples as disease.
- Treating SDMA as a required OECD 408 replacement for creatinine rather than an adjunct GFR marker.
In a repeat-dose dog study, which statement correctly contrasts leakage enzymes with induction enzymes?
High-dose rats show about a 10-fold increase in ALT and GLDH, ALP and bilirubin within concurrent-control ranges, CK unchanged, and centrilobular hepatocellular necrosis. What is the most coherent interpretation?
The first terminal chemistry tube is grossly hemolyzed and shows high AST and potassium. A second, nonhemolyzed tube from the same animal has AST and potassium within concurrent-control ranges; ALT, GLDH, and CK were never increased. What is the best interpretation?