Diagnostic Enzymology and Hepatic System

What are the 3 main function of bile?

eliminate waste (excretory)

facilitate digestion of lipids

provision of buffers

What percentage of bile acids is recycled?

95%

resorbed in colon

What are bile acids produced from?

cholesterol

Describe bilirubin metabolism.

aged red cells are phagocytized primarily in the spleen. Hemoglobin splits into globin and heme which further splits into iron and a substance that gets converted to unconjugated or free bilirubin,

Unconjugated bilirubin is released from macrophages and transported through blood bound to albumin as unconjugated is not water soluble. Brought to liver, released and conjugated with gluccuronic acid. Excreted into intestine.

Anaerobic bacterial enzymes in intestine form urobilinogen and some of which is absorbed and excreted in urine. The rest is reduced to stercobilin (orange brown pigment) and leaves in feces.

True or false:

When there is significant interference due to the presence of lipemia (turbidity), hemolysis or bilirubin (icterus) many results on a serum chemistry panel can be affected, including serum enzyme activity results.

true

The “__________” of plasma affects what enzymes are able to be measured. If a tissue releases enzymes in very small quantities, they may not be significant in blood due to the dilution from the plasma (e.g. enzymes from an adrenal gland).

dilution effect

___________ enzymes are present within the cytosol, organelles or both. Enzymes are at very high levels within a cell, and thus even with very mild cell injury, the increased enzyme activity in the serum can be a sensitive indicator for injury.

Leakage

____________ enzymes are attached to cell membranes and serum activities typically increase due to increased production of these enzymes by action of an inducer (e.g. phenobarbital for alkaline phosphatase

Induction

Increased serum activity of induction enzymes takes ____________to occur as compared to leakage enzymes as induction enzymes depend on increased production.

longer (days)

___________ are multiple forms of the same enzyme that are able to catalyze the same enzymatic reaction but are different structurally as they are encoded by separate structural genes. If there is a post-translational modification of the original gene product that causes the different enzyme structure, it is known as an _______.

Isoenzymes

isoform

What are 5 hepatic leakage enzymes that we evaluate?

• alanine aminotransferase (ALT)

• Aspartate aminotransferase (AST)

• Sorbitol dehydrogenase (SDH)

• Glutamate dehydrogenase (GDH, GMD, and other names)

• Lactate dehydrogenase (LHD, or LD)

Is ALT liver specific? What species do we use this in?

• mostly however increases can be seen with severe muscle injury

• dogs and cats but is of little value in large animal species

In _____________ liver cell injury, serum enzyme activity will often only be mildly elevated and while the serum enzyme activity will be an accurate indication of the daily loss or rate of loss of hepatocytes

chronic progressive

therefore, ALT is more useful in acute injury

Is AST liver specific? What species if the half life longest in?

no—> found in liver and muscle of all species.

horses

NOTE: ALT is used more in SA but in LA, we can use AST in combo with others

Is SDH liver specific? When do we use this?

Yes, found in cytoplasm of liver cells.

LA’s but has a short half life so may return to normal in 4-5d

Where is GDH found?

mitochondrial enzyme in high amount in hepatocytes. Liver specific but no advantage over ALT for cats and dogs.

Is LDH used often?

no—> not routine. It is found in cytoplasm of many cells so increased activity is not specific to hepatic injury

What four values are you measuring for cholestasis indicators?

• Alkaline phosphate (ALP)

• GGT

• bilirubin

• bile acids

Where is ALP found and how are they made?

bound to cell membranes

induced (not leaked)

What are the two main ALP isoenzymes? And how/why do they form?

• intestinal ALP isoenzyme forms corticosteroid induced ALP isoform (Ci-ALP) which is specific to canines

• tissue non-specifc ALP isoenzymes form the liver ALP (L-ALP), bone ALP (B-ALP), etc.

What are 5 reasons you may see increased serum ALP activity?

• cholestasis (bc increased L-ALP)

• induction by drugs/hormones (steroids and anticonvulsants, important in dogs)

• increased osteoblastic activity (growth, hyperparathyroidism, bone healing)

• benign familial hyperphosphatasemia (Huskeys and Terriers)

• neoplasms

Two drugs that can increase ALP? Important in what species?

anti-convulsants

steroids

canines

True or false:

Colostrum has no effect on ALP levels

False: increased in first 3 days