Which porphyrin is excreted primarily in urine?

• A. URO
• B. PROTO
• C. COPRO
• D. BOTH URO AND COPRO

Answer: (A)

Porphyrin excretion follows solubility: the more water-soluble porphyrins are cleared by the kidneys into urine, while the less soluble ones are secreted into bile and eliminated in feces. Uroporphyrin has more carboxyl groups, making it highly polar and water-soluble, so it is readily excreted in urine. Protoporphyrin is relatively lipophilic and is mainly eliminated via the biliary route into feces. Coproporphyrin sits in between, with excretion occurring through both urine and feces, but not predominantly in urine like uroporphyrin. Therefore, uroporphyrin is excreted primarily in urine. Clinically, increased urinary uroporphyrin helps indicate certain porphyrias such as porphyria cutanea tarda.