Chemistry and Biology of Proteins - F. Haurowitz 1953

Conjugated Proteins
Compounds of proteins with porphyrins and bile pigments

The normal concentration of Porphyrins in Body Fluids is very low; however, it can increase significantly under certain pathological conditions. Porphyrins are typically bound to plasma albumins [230]. The green pigment chloroma (a pigment found in Bone Marrow lymph tumors) is protoporphyrin [231]. This pigment is bound to insoluble Proteins and therefore cannot be extracted with diethyl ether.

Bilirubin, an elevated concentration of which in Blood Plasma serves as a symptom of jaundice, is bound to serum albumin [232]. Each serum albumin molecule is capable of binding 3 molecules of bilirubin [233].

Electrophoretic studies have demonstrated that not only direct bilirubin, but also the so-called indirect bilirubin migrates together with the albumin and a-globulin fractions [234]. Indirect bilirubin reacts with diazobenzenesulfonic acid only after ethyl alcohol has been added to its solution.

The dark brown color of blood serum and feces, observed in relatively rare cases of myodystrophy, is due to myobilin, a conjugated protein containing mesobilifuscin [235]. The latter is a dipyrrylmethane formed during The breakdown of Hemoglobin or its derivatives.

Conjugated Proteins containing bilirubin derivatives have also been found in red Algae (Erythrophyceae). Treating them with an alkaline solution of methyl alcohol yields mesobilirubin and mesobiliverdin [236]. The in vivo breakdown of hemoglobin produces green-colored Chromoproteins. The first intermediate product in hemoglobin degradation is choleglobin, which consists of globin and a prosthetic group that is a compound of ferric iron and biliverdin [237]. Bilirubin contains four pyrrole rings linearly connected to each other. In the formula below, M designates a methyl group (—СН₃), V a vinyl group (—СН=СН₂), and P propionic acid (—СН₂СН₂СООН):

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Upon Denaturation of choleglobin, cholehemochromogen is formed—a complex of denatured globin and a compound of biliverdin with ferric iron [237]. Biliverdin is cleaved off upon Treatment with acids. Similar green chromoproteins, representing open chains of tetrapyrrole rings, can be obtained in vitro by treating hemoglobin with cyanides and hydrogen peroxide. These compounds are designated as verdoglobins and pseudohemoglobins, and they are frequently detected in individuals receiving sulfonamides [238]. While the iron in the porphyrin ring of hemoglobin is bound so tightly that it is not released even upon exposure to Hydrochloric acid, the iron incorporated into verdoglobins is readily split off by dilute hydrochloric acid. The "inactive" hemoglobin present in small amounts in normal blood is likely identical to verdoglobin. Its concentration is approximately 1.3% [239]. The concentration of verdoglobin in the blood can be determined spectrophotometrically [240]. Small amounts of a green pigment similar to choleglobin have been found in the ROOT nodules of legumes [241].