Porphyrin profile and chelator techniques in the assessment of mercury exposure and effects

Heavy metal contamination is wide spread globally as a result of mining and other industrial processes. Health effects end points, or biomarkers, combined with accurate estimates of target tissue metal burdens, are necessary to more precisely determine the risk posed to wildlife and humans. An optimal assessment tool would be one that non-lethally assesses both heavy metal health effects and metal burdens at target site soft tissues, allowing researchers to sample a given animal multiple times in a given year, use more animals than would otherwise be permitted (increasing the statistical power to the local population level), and study animals for which access is generally restricted, such as threatened or endangered species.

A technique was developed using porphyrin profile alterations as a marker of biochemical effects, and the water-soluble chelator DMPS to approximate soft tissue mercury burdens through urinary depuration. A collection of urine is taken, DMPS is administered, and a subsequent urine sample is collected. The pre-chelation urinary porphyrin concentrations are indicative of existing health status, while the post-chelation urinary mercury residues approximate the soft tissue mercury burden.

Preliminary studies with the prairie vole {Microtus ochrogastor) and deer mice {Peromyscus maniculatus) indicated the vole was the preferred test species based on responsiveness and urine volume generated. Methyl-mercury in drinking water (5 ppm) led to significant, 2-3 fold increases in urinary 5- and 4-CP. In voles exposed to methylmercury in their drinking water sub-chronically, renal residues increased in a dose dependent manner, with a plateau in concentration before, or during, the first three weeks of exposure. Urinary and renal porphyrins were analyzed and were highly variable.

A field trial was performed on the Carson River mercury site near Virginia City, Nevada. Deer mice {Peromyscus maniculatus) and mountain voles {Microtus montanus) captured on this site had elevated mercury and porphyrin concentration when compared to reference site animals from the east fork of the Carson River. In deer mice, renal 4- carboxylporphyrin was significantly correlated to renal mercury resides (r = 0.4294, p = 0.0001, n = 38), while vole urinary pre-chelation 5-carboxylporphyrin was significantly related to urinary post-chelation organic mercury concentrations (r = 0.5870, p = 0.0001, n = 32).

These findings support the continued study and application of the combined porphyrin profile/DMPS challenge technique to wildlife inhabiting metal contaminated sites. With further experimentation, this tool offers the capability to better assess heavy metal contamination and effects non-lethally in wildlife species under a variety of exposure scenarios.