The isomer ratios of urinary coproporphyrins I--IV are pH-dependent.

The percentage of porphyrinogens as related to total porphyrin excretion was determined in the urine of healthy subjects. Acidic urines (pH 5.0 to 5.9) contained 62.9 +/- 10.7% (means +/- s, N = 11) porphyrinogens, whereas in neutral urines (pH 6.0 to 7.2) a somewhat lower percentage (51.2 +/- 15.3%, N = 11) was detected. However, there was no significant difference between the mean porphyrinogen contents of acidic and neutral urines. Evidence was found for a previously unreported pH-dependent influence on the isomer ratios of urinary coproporphyrins I and III. Acidic urines (N = 18) from healthy subjects showed significantly higher percentages of isomer I (27.1 +/- 6.4%), isomer II (2.7 +/- 1.1%), and isomer IV (5.0 +/- 1.3%) as compared to respective values from neutral urines (22.2 +/- 5.1% isomer I, 0.6 +/- 0.6% isomer II, and 1.5 +/- 1.3% isomer IV; N = 16, p less than 0.001). Conversely, the percentage of isomer III was markedly lower in acidic urines than in neutral urines (65.1 +/- 7.9% vs. 75.9 +/- 5.4%; p less than 0.001). The same relationship was confirmed in an individual subject by analysis of a series of urines (N = 13) with pH values ranging from 5.4 to 7.3. These results point to the possibility that the atypical coproporphyrin isomers II and IV are predominantly formed by an increased isomerization rate of coproporphyrinogens under acidic intravesical conditions.