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Porphyrinogens are cyclic tetrapyrroles- that is, they are a class of biochemical compounds that are composed of four pyrrole groups connected in a ring-like structure. Uroporphyrinogens are porphyrinogens that also contain a propionic acid ("P" group) and acetic acid ("A" group) side group connected to each pyrrole in the macrocyclic core (8 side groups in total, 4 Ps and 4 As). There are four variants of uroporphyrinogen (uroporphyrinogen I-IV), which are differentiated from each other by the arrangement of the P and A side groups. Uroporphyrinogen I and III are naturally occurring compounds, while uroporphyrinogen II and IV do not occur in nature and are only obtainable synthetically. Uroporphyrinogen III is of particular importance in the field of medicine due to its presence in the human body as an intermediate of the biosynthetic pathway that produces heme.[1]

The group of diseases referred to as the porphyrias are metabolic disorders- they are caused by an abnormality in the function of the enzymes involved in the biosynthesis of heme. The anomaly usually manifests as an alteration in the activity (increased or decreased) of the enzyme, and most of the porphyrias have a genetic mutation that is responsible for the dysfunction. Porphyria cutanea tarda (PCT) is the lone exception, as it can develop in the absence of a gene mutation and is, in fact, most often caused by an acquired inhibitor (around 2/3 of cases).[8] Also, most genetic mutations occur in the gene that encodes the respective synthetic heme enzyme, with exceptions including rare instances of congenital erythropoietic porphyria (CEP), in which the causative mutation is in a transcription factor that controls the expression of the enzyme uroporphyrinogen III synthase.[9] Thus, any of the porphyrias may exhibit a predictable Mendelian genetic inheritance pattern as the genes for all the synthetic heme enzymes (or genes related to their expression) are found within the cellular genome (none of the genes are mitochondrial). The clinical presentation of the different porphyrias depends on the noxious metabolites that accumulate, their sites of accumulation, and the method by which those metabolites get excreted. Depending on the solubility of the compounds in different cellular components, they may accumulate in different areas of the body and be excreted by the liver (in bile and feces) or by the kidneys (in urine). This excretion is useful as levels of specific porphyrins can be measured in urine and feces to aid in the diagnosis. Also, certain symptoms such as cutaneous blistering or neurovisceral manifestations (abdominal pain, vomiting, weakness, psychiatric changes) are typically seen in certain porphyrias depending on the accumulation of porphyrins in specific areas such as the skin or nervous system. Knowledge of this can be used to help aid in forming the differential diagnosis of a patient with suspected porphyria.[10]