Giving porphyrins a good name

In uroporphyrins there are two types of substituerrt,-CH2.C02H (A) and-CH2 CH2-C02H (P). This gives fgur Bi isontiers depending on which p-positions A and P occupy. 

During porphyrin biosynthesis -A groups are converted to -Me and two of the -P groups are converted to -V 

The Fischer system is straightforward for naming simple porphyrins, but as the complexity of a porphyrin derivative increases, with different substituents on the p- and meso-positions, the system becomes unwieldy and even contradictory (systems of nomenclature have to be self-consistent). 

Mesoporphyrin IX - Fischer nomenclature 2,4-diethyl-1,3,5,8-tetramethylporphyrin-6,7-dipropionic acid 

Mesoporphyrin IX - Systematic nomenclature 7,12-diethyl-3,8,13,17-tetramethylporphyrin-2,18-dipropionic acid 

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