Harpers

282 / CHAPTER 32MH 2 CMINHEOHOHMMH 2 CMINHEHHOHOHHMHMH 2 CMINHEHHOHOHHMHNHHNNHHNNHNPHNH 2 C CH 2EPHNH 2 C CH 2EPHNHC CH 2EPMMesobilirubinogen(C 33 H 44 O 6 N 4 )PMStercobilinogen(L-Urobilinogen)PStercobilin(L-Urobilin)MFigure 32–16.Structure of some bile pigments.it diffuses into the tissues, which then become yellow.That condition is called jaundice or icterus.In clinical studies of jaundice, measurement ofbilirubin in the serum is of great value. A method forquantitatively assaying the bilirubin content of theserum was first devised by van den Bergh by applicationof Ehrlich’s test for bilirubin in urine. The Ehrlich reactionis based on the coupling of diazotized sulfanilicacid (Ehrlich’s diazo reagent) and bilirubin to producea reddish-purple azo compound. In the original procedureas described by Ehrlich, methanol was used toprovide a solution in which both bilirubin and thediazo regent were soluble. Van den Bergh inadvertentlyomitted the methanol on an occasion when assay of bilepigment in human bile was being attempted. To hissurprise, normal development of the color occurred “directly.”This form of bilirubin that would react withoutthe addition of methanol was thus termed “directreacting.”It was then found that this same direct reactionwould also occur in serum from cases of jaundicedue to biliary obstruction. However, it was still necessaryto add methanol to detect bilirubin in normalserum or that which was present in excess in serumfrom cases of hemolytic jaundice where no evidence ofobstruction was to be found. To that form of bilirubinwhich could be measured only after the addition ofmethanol, the term “indirect-reacting” was applied.It was subsequently discovered that the indirectbilirubin is “free” (unconjugated) bilirubin en route tothe liver from the reticuloendothelial tissues, where thebilirubin was originally produced by the breakdown ofheme porphyrins. Since this bilirubin is not water-soluble,it requires methanol to initiate coupling with thediazo reagent. In the liver, the free bilirubin becomesconjugated with glucuronic acid, and the conjugate,bilirubin glucuronide, can then be excreted into thebile. Furthermore, conjugated bilirubin, being watersoluble,can react directly with the diazo reagent, sothat the “direct bilirubin” of van den Bergh is actually abilirubin conjugate (bilirubin glucuronide).Depending on the type of bilirubin present inplasma—ie, unconjugated or conjugated—hyperbilirubinemiamay be classified as retention hyperbilirubinemia,due to overproduction, or regurgitation hyperbilirubinemia,due to reflux into the bloodstreambecause of biliary obstruction.Because of its hydrophobicity, only unconjugatedbilirubin can cross the blood-brain barrier into the centralnervous system; thus, encephalopathy due to hyperbilirubinemia(kernicterus) can occur only in connectionwith unconjugated bilirubin, as found in retentionhyperbilirubinemia. On the other hand, because of itswater-solubility, only conjugated bilirubin can appearin urine. Accordingly, choluric jaundice (choluria isthe presence of bile pigments in the urine) occurs onlyin regurgitation hyperbilirubinemia, and acholuricjaundice occurs only in the presence of an excess of unconjugatedbilirubin.Elevated Amounts of UnconjugatedBilirubin in Blood Occur in a Numberof ConditionsA. HEMOLYTIC ANEMIASHemolytic anemias are important causes of unconjugatedhyperbilirubinemia, though unconjugated hyperbilirubinemiais usually only slight (< 4 mg/dL; < 68.4µmol/L) even in the event of extensive hemolysis becauseof the healthy liver’s large capacity for handlingbilirubin.B. NEONATAL “PHYSIOLOGIC JAUNDICE”This transient condition is the most common cause ofunconjugated hyperbilirubinemia. It results from an ac-