Bilirubin

Description

Bilirubin is a pigment formed from the haem groups of haemoglobin molecules. The enzymatic conversion occurs within cells of the monocyte-phagocyte system (MPS) when haemoglobin is released by the degradation of red blood cells. This unconjugated bilirubin is not water soluble and it is transported in plasma bound to albumin. It is taken up into hepatocytes where it is conjugated with glucuronic acid. The resultant conjugated bilirubin is water soluble and it is secreted into bile which then moves into the small intestine.

The conjugated bilirubin is degraded to urobilinogen by gastro-intestinal bacteria and a small proportion of this product is reabsorbed and excreted in the urine. The remaining urobilinogen is further degraded to stercobilin, a brown pigment which contributes to the colour of faeces. Therefore, in animals with complete biliary obstruction, urobilinogen is absent from the urine and the faeces have a white/grey 'acholic' colour due to the absence of stercobilin. The latter alteration in faecal colour also results from steatorrhoea.

Small quantities of conjugated bilirubin are found in the urine of normal dogs because it has a low renal threshold.

Hyperbilirubinaemia

Bilirubin increases when the production of bilirubin exceeds the ability of the liver to recover unconjugated bilirubin from the bloodstream and process it, or when clearance of the conjugated form is altered by impaired hepatocyte processes or obstruction to the discharge of bilirubin into the intestines. Clinically, this means that hyperbilirubinaemia can be cased by heamolysis or choleostasis.

The Van de Berg test can be used to distinguish conjugated from unconjugated bilirubin. Plasma from an icteric animal is treated with an aqueous solution of the reagent diazotised sulphanilic acid and this produces a red-purple colour reaction. The intensity of this colour is directly proportional to the amount of water soluble (conjugated ) bilirubin in the sample. Further addition of alcohol intensifies the colour if there is non-water soluble (unconjugated) bilirubin also present. The intensified colour is directly proportional to the total amount of bilirubin present in the sample and the difference between the two readings gives the amount of unconjugated bilirubin in the sample.

Clinical Significance

Icterus refers to the staining of tissues by bilirubin pigment or bilirubin complexes, a phenomenon that is clinically evident on examination of the sclera and mucous membranes.