Coproporphyrin assay

The first porphyrin intermediate of the biosynthetic pathway is uroporphyrinogen, which is stepwise decarboxylated by uroporphyrinogen decarboxylase to heptacarboxy-, hexacarboxy-, pentacarboxy-, and coproporphyrinogen. This latter compound proceeds, as indicated in Fig. 7.3.1, to protoporphyrinogen and protoporphyrin. The oxidized uroporphyrin and its decarboxylation products up to coproporphyrin are assayed in urine. Coproporphyrin and the further downstream intermediaries can be recovered from feces as described below. 

Increased porphyrins in clear fluid such as urine may be detected directly by their pink fluorescence if exposed to long ultraviolet (Fig. 7.3.2). The specificity of this screening assay may be improved if porphyrins are extracted by talcum [8]. These isolated porphyrins may be quantified using a spectrofluorimeter. As different porphyrias show specific excretion patterns, separation of the main porphyrins is desirable. The formerly used fractionated extraction enabled to separate the uroporphyrin fraction from the coproporphyrin fraction. In addition to uroporphyrin, the first fraction includes heptacarboxy- and part of hexacarboxyporphyrins, and in addition to coproporphyrin, the second fraction includes part of hexacarboxy- and pentacar-boxyporphyrins. Later on, thin-layer chromatography of methylester derivatives is used. 

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