Coproporphyrin isomers II and IV are normal constituents of human urine.

We describe for the first time the detection of small amounts of the atypical coproporphyrin isomers II and IV in urine from healthy subjects. Efficient sample preparation procedures together with a highly selective isocratic ion-pair high-performance liquid chromatographic method enabled the determination of these unexpected compounds. Formation of the atypical coproporphyrin isomers was shown to result mostly from non-enzymatic isomerization at the coproporphyrinogen level within the human body, whereas only traces were formed in vitro during collection and storage of the urine samples under the conditions applied.