PRINCIPLES OF TOXICOLOGY

126 HEPATOTOXICITY: TOXIC EFFECTS ON THE LIVER
and excretion of bilirubin results in its accumulation in the blood, leading to jaundice. Serum bilirubin
concentrations may be elevated from acute hepatocellular injury, cholestatic injury, or biliary obstruction.
This test is always included among the battery of tests to assess liver function clinically, although
it is not a particularly sensitive test for acute injury.
4. Dye Clearance Tests. These tests involve administration of a dye that is cleared by the liver and
measurement of its rate of disappearance from the blood. Delayed clearance is interpreted as evidence
of liver injury. One such dye is sulfobromophthalein (Bromsulphalein; or BSP). Clearance of BSP
from the blood is dependent on its active transport into liver cells, conjugation with glutathione, and
then active transport into the bile. Conceivably, disruption of any of these processes could result in
delayed clearance, although the biliary excretion step is regarded as most critical. The test consists of
administering a dose of the dye intravenously and measuring its concentration in blood spectrophotometrically
over time. Another dye used for this purpose is indocyanine green (ICG). Unlike BSP,
ICG is excreted into the bile without conjugation. Following an intravenous dose, the disappearance
of ICG from blood can be measured with repeated blood samples or noninvasively by ear densitometry.
The dye tests, although well established, are seldom used clinically.
5. Drug Clearance Tests. This test relies on the principle that liver injury will result in impaired
biotransformation. The biotransformation capacity of the liver is assessed by following the rate of
elimination of a test drug whose clearance from blood is dependent on hepatic metabolism (i.e., a drug
for which other elimination processes, such as renal excretion, are insignificant). A test drug such as
antipyrine, aminopyrine, or caffeine is administered, and its rate of disappearance from blood is
followed over time through serial blood sampling. This rate is compared with a value considered
“normal” to determine whether impaired biotransformation exists. This can also be used to test for
hepatic enzyme induction, in which the rate of elimination from blood would be increased, rather than
decreased as in liver injury. This test is primarily used for research purposes.
6. Measurement of Hepatic Enzymes in Serum. Cells undergoing acute degeneration and injury
will often release intracellular proteins and other macromolecules into blood. The detection of these
substances in blood above normal, baseline levels signals cytotoxicity. This is true for any cell type,
and in order for the presence of intracellular proteins in blood to be diagnostic for any particular type
of cell injury (e.g., liver toxicity versus renal toxicity versus cardiotoxicity), the proteins must be
associated rather specifically with a target organ or tissue. Fortunately, several proteins are found
primarily in hepatocytes, and their presence in blood in elevated levels is the basis for some of the most
commonly used tests for hepatotoxicity. Table 5.4 shows many of the most common proteins measured
in these tests. The reader will note that all of these proteins are enzymes. This is not a coincidence.
While any intracellular protein specific to the liver would be useful theoretically, enzymes are proteins
that can be measured specifically (by measuring the rate of their particular enzyme activity) using
TABLE 5.4 Serum Enzyme Indicators of Hepatotoxicity
Enzyme Acronym Comments
Alanine aminotransferase ALT Found mainly in the liver; increase reflects primarily
hepatocellular damage
Aspartate aminotransferase AST Less specific to the liver than ALT; increase reflects primarily
hepatocellular damage
Alkaline phosphatase ALP Increases reflect primarily cholestatic injury
γ-Glutamyl transferase;
GGTP Increases reflect primarily cholestatic injury, although
γ-glutamyltranspeptidase
elevated in hepatocellular damage as well
5′-Nucleotidase 5′ND Increases reflect primarily cholestatic injury
Sorbitol dehydrogenase SDH High specificity for liver; increase reflects primarily
hepatocellular damage
Ornithine carbamoyltransferase OCT High specificity for liver; increase reflects primarily
hepatocellular damage