Bilirubin and Liver Enzymes

Bilirubin:
Bilirubin gets complicated! I’m going to teach you how to think your way through bilirubin problems by having a deeper understanding of the pathways.

Bilirubin is a molecule derived from RBC turnover and more specifically, heme turnover. As talked about earlier, RBCs have a lifespan of about 120 days, and about 1% of RBCs are removed daily. Most RBCs are phagocytosed in the spleen by macrophages.

Bilirubin is a waste product. Since the body wants to recycle and reuse the iron from the heme molecule, it needs to get it out of the porphyrin ring.

The molecule is opened up using the enzyme heme oxygenase. Ok, so now the iron has been let out, now what? We’re left with an opened ring. We got the candy out and now we’re left with the wrapper. This wrapper has a name – biliverdin.

Now it’s time to throw away the wrapper! Biliverdin is quickly converted into bilirubin and it heads back to the blood stream where unconjugated bilirubin joins up with albumin. Albumin carries the loosely bound bilirubin to the liver (albumin is kind of like the bus of the bloodstream). Bilirubin is lipid soluble so being bound to albumin prevents it from entering into cells and causing damage. Since bilirubin is not water soluble it doesn’t want to exit through the kidneys. Instead, it exits through the bile via the liver. Once across the hepatocyte membrane, bilirubin is rapidly conjugated by glucuronic acid. Conjugated bilirubin then leaves the hepatocyte via active transport and enters the bile and then the intestine where it is converted to urobilinogen. About 80% of urobilinogen exits at this point, and it’s what gives feces its brown color. Gross! Ok what about the other 20% of urobilinogen? It gets reabsorbed into the liver where most of that goes back to the bile but a small percentage goes back to the plasma and exits through the urine.

Why is bilirubin a good diagnostic tool? Bilirubin has different forms depending on where it is, unconjugated vs conjugated. It can be used to decipher what may be going wrong in a patient. In the lab total bilirubin and direct bilirubin are measured (direct bilirubin is a representation of conjugated bilirubin). Indirect bilirubin which represents unconjugated bilirubin can be inferred from a simple calculation.

Bilirubin conditions are best understood when broken down into three categories: pre-hepatic, hepatic, and post-hepatic.

Pre-hepatic:
Pre-hepatic examples are hemolysis and ineffective erythropoiesis (red cells aren’t formed properly leaving unused heme). These conditions will cause an increase in plasma unconjugated bilirubin, and an increase in urine urobilinogen. Why is this? Plasma unconjugated bilirubin is increased due to the influx of heme destruction. The conversion of unconjugated bilirubin to conjugated bilirubin is a rate limiting step which only goes so fast, so the conjugated bilirubin remains at normal levels. Why is urine urobilinogen elevated? Since the factory is busier, more conjugated bilirubin is excreted which causes more of that small percentage of urobilinogen to enter the plasma causing an increase in the urine.

Hepatic:
Hepatic examples include cirrhosis and viral hepatitis. Both of these conditions will give you inflamed hepatocytes amongst other liver problems causing the liver machinery to not function properly. Unconjugated bilirubin will be increased because the conjugation process in the liver isn’t working at normal capacity. The unconjugated bilirubin goes back into the blood stream. Conjugated bilirubin will be increased due to decreased excretion related to the liver. The conjugated bilirubin will also backup into the bloodstream and some of this bilirubin, since it is conjugated can, and will, exit through the kidneys causing an increase in urine bilirubin. Urine urobilinogen will also be increased because the ~20% fraction that is reabsorbed by the liver isn’t reabsorbed properly and is sent into circulation. This urobilinogen is excreted by the kidneys increasing urine urobilinogen.

Post-hepatic:
Post-hepatic examples include problems with the flow of bile from the liver (cholestasis). Conjugated bilirubin can’t get to where it needs to go and it backs up into the bloodstream. This causes an increase in conjugated bilirubin and also urine bilirubin just like in hepatic problems. The conjugation process and other liver functions are fine here so there’s no increase in unconjugated bilirubin. The decrease in urobilinogen is due to the fact that conjugated bilirubin can’t get through to create urobilinogen. This can create symptoms like a clay colored stool.

Newborns:
Newborns are susceptible to jaundice, and to get rid of it they will shine blue light on them. It’s somewhat common, and what’s happening is unconjugated bilirubin is exceeding albumin binding capacity and since unconjugated bilirubin is lipid soluble it can cross things like the immature blood brain barrier in newborns causing damage to  brain cells (kernicterus). Most cases don’t progress that far because simply applying the light can change the confirmation of bilirubin making it water soluble so it can be eliminated in the urine. Things that can cause increased unconjugated bilirubin in neonates is high RBC turnover and immature enzymes that conjugate bilirubin.

Study Tip
There’s a simple table to remember which can help determine which type of condition is associated with bilirubin test results.

	Plasma		Urine
	unconjugated	conjugated	bilirubin	urobilinogen
Pre-hepatic – hemolytic anemia	↑	N	0	↑	UNOU
Hepatic – cirrhosis, viral hepatitis	↑	↑	0 or ↑	↑	UUOUU
Post-hepatic – obstructive	N	↑	↑	↓	NUUD
Newborns	↑	N	0	0	UNOO

This table is fairly easy to recall by using the codes at the far right of the table. They seem a little odd but they will eventually stick. U represents ↑, D represents ↓, N represents normal, and 0 represents none.

In a hepatic condition like cirrhosis, bilirubin may not be able to enter the bile because the machinery is broken. If this happens, it enters the plasma. The kidneys will then filter it into the urine, also causing an increased urine urobilinogen.

Liver enzymes:
The aminotransferases, alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are not specific to liver function but are markers for liver health. ALT will show the greatest specificity for hepatocellular damage.

Alkaline phosphatase (ALP) is a marker that is elevated in bone diseases like Paget’s disease and rickets. ALP is also a marker for extrahepatic biliary obstruction. The biles got to flow!

Gamma-glutamyltransferase (GGT) is a more specific hepatobiliary disease marker than ALP. GGT is also elevated in hepatitis as well as alcoholism.

5’ Nucleotidase (5’NT) is a random liver marker discovered many years ago to be fairly specific for biliary obstruction.

In hepatitis, AST and ALT are elevated; in biliary obstruction, ALP and 5’NT are elevated. GGT and bilirubin are elevated in both hepatitis and biliary obstruction.

Study Tip
Remember liver marker conditions using the anagrams Pat Sat Lat (Hepatitis, AST, ALT), and 5 Plant obstruction (ALP and 5’NT increased in biliary obstruction)