Sixty years of conjecture over a urinary biomarker: a step closer to understanding the proposed link between anxiety and urinary pyrroles

Abstract

Objective: For over 60 years there has been conjecture about the identity of an Ehrlich's test positive pyrrole (Mauve Factor) reputed to be a biomarker for psychological disorders, including anxiety. We reviewed studies that attempt to identify Mauve Factor and subjected authentic standards of the 2 main candidates, kryptopyrrole and hydroxypyrrole, to the Ehrlich's reaction.

Methods: Modified Ehrlich's test for kryptopyrrole and hydroxypyrrole were applied to urine samples from 10 volunteers, anxious and nonanxious.

Results: Based on the mechanistic chemistry of Ehrlich's reaction and reactions of the 2 compounds, Mauve Factor cannot be hydroxypyrrole. Analyses of urine samples from volunteers, identified by the Generalized Anxiety Disorder - 7 item scale (GAD-7 ≥10; n = 5) and control urine samples (GAD-7 <10; n = 5) using a kryptopyrrole calibration graph, show that concentrations are similar in both groups.

Conclusion: Kryptopyrrole may be the elusive Mauve Factor. Its possible origin from stercobilin via gut microbiome-mediated metabolism, its link to gut-mediated neurological effects via γ-aminobutyric acid (GABA) receptors, and its predicted interaction with Zn2+ and consequent impact on zinc homeostasis are discussed. The GAD-7 scale does not differentiate between state and trait anxiety and as such, the minimal difference in pyrrole levels between volunteer groups requires further study.

Keywords: Mauve Factor; biomarker for anxiety; generalized anxiety disorder; hydroxypyrrole; kryptopyrrole; urinary biomarker.

Figure 1.

Proposed molecular structures of Mauve Factor. Left: 2,4-dimethyl-3-ethylpyrrole (kryptopyrrole; Irvine et al). Right: 2,4-dimethyl-3-ethyl-2-hydroxy-2,5-dihydropyrrol-2-one (hydroxypyrrole [HPL]; Irvine). The pyrrole ring numbering system and α- and β-carbons are also shown.

Figure 2.

A dipyrrolic fragment of bilirubin from the catabolism of heme proposed by Brown and King.

Figure 3.

Ehrlich’s reaction: the β-carbon of indole (A) reacts with 4-dimethylaminobenzaldehyde (DMAB) (B) to form purple β-bis(indolyl)dimethylaminobenzyl methane (C).

Figure 4.

Reaction of hydroxypyrrole (HPL) and kryptopyrrole with modified Ehrlich’s reagent. This clearly shows that HPL does not react with 4-dimethylaminobenzaldehyde (DMAB) and that kryptopyrrole reacts in a concentration-related manner. Results are shown as mean ± SD (n = 3).

Figure 5.

Postulated structure of the pyridoxal phosphate/kryptopyrrole-zinc chelate (left) and possible, but unlikely, zinc-kryptopyrrole complex (right).

Figure 6.

Stercobilin: the two 3-ethyl-4-methyl-3,5-dihydropyrrol-2-one moieties at each end of the molecule have significant structural similarities to hydroxypyrrole (HPL) and kryptopyrrole (see FIGURE 1).