High bilirubin levels might be a symptom of liver disease in adults or older children, but it can be common in newborns. Bilirubin is a naturally occurring organic molecule that appears in bile, blood, urine and body fluids as a result of the breakdown of red blood cells. It is classified as a pigment having the multiple conjugated double bonds that absorb particular wavelengths of light resulting in colored molecules. The yellow-green of high bilirubin levels gives bruises, jaundiced babies, urine and bile their characteristic color. Similar to pigments used by plants to sense light or by certain algae to capture light energy, bilirubin consists of an open chain of four five-member, nitrogen-containing, pyrollic rings.
The heme portion of hemoglobin, the oxygen-carrying molecule in blood, consists of a large ring, called a porphyrin ring made of four smaller pyrollic rings. In the center of the large ring lies an iron atom. Its state of ionization determines the degree of attraction to oxygen molecules. Upon the death of the red blood cell, the porphyrin ring is broken, the iron recovered for recycling, and the remaining useless open chain residual molecule of bilirubin is released into the surroundings. It is a waste product to be removed.
The released bilirubin in its unconjugated form is not water soluble but is bound with albumin, the predominant plasma protein, for transport to the liver where it is conjugated with glucuronic acid. The majority of this conjugated form is concentrated in bile and released into the small intestine. A small amount might be discharged in urine. The presence of increased bilirubin in the urine in its conjugated and unconjugated forms serves as an indicator of possible liver damage or disease.
The blood detoxification activity of the liver is one of the last to be fully functional in newborns, because their mothers' digestive systems performed this task for them. Within two to five days after birth, a newborn’s liver must take over the filtration of the blood. In the interim, the baby might experience a high bilirubin count to the degree that the whites of the eyes and the skin have a yellow tint.. This is of concern as the blood brain barrier is not fully established in the infant, and high bilirubin levels can leave deposits in the brain and cause neuronal defects. Light therapy in which the baby is exposed to natural or artificial light sources for specified periods of time causes the light-sensitive bilirubin molecule to isomerize, a changing of its geometrical configuration, to a water-soluble form and be eliminated from the body in urine.
In adults, moderately increased bilirubin levels are generally not harmful. Five to 10 percent of the population is thought to have Gilbert’s syndrome, which causes a lower activity in the enzyme that conjugates the bilirubin. Other causes of high bilirubin levels include the use of antipsychotic drugs and chemotherapy. Seriously high bilirubin levels are associated with bile duct obstruction, cirrhosis or other liver disease.
