ORNITHINE DECARBOXYLASE ASSAY IN HUMAN COLORECTAL MUCOSA - METHODOLOGIC ISSUES OF IMPORTANCE TO QUALITY-CONTROL

Ornithine decarboxylase may be a useful biomarker for risk of neoplasia in colorectal tissues. Investigators have reported enzyme activities varying by as much as 10- to 20-fold using variations of the usual (CO2)-C-14 release assay. We have examined the effect of different methodologic factors on calculated ornithine decarboxylase activity. Major effects on the assay result (>20% change) were produced by: (1) use of Tris vs. phosphate buffer, the former yielding 1.5- to 4-fold greater activity; (2) protein content of the reaction mixture with significant error if <50 mug; (3) use of alpha-difluoro-methylornithine-inhibited blank versus buffer-only blank. Other changes in assay conditions, including addition of sucrose, detergent, protease inhibitors, specific activity of C-14-ornithine, the nature of the trapping agent used, and incorporation of a sonication step, did not have a significant effect on ODC quantification (less-than-or-equal-to 20%). Thus, seemingly minor variations in assay conditions can greatly affect the results, which may provide a partial explanation for the variability of ODC activities reported in the literature. Strict quality control measures are mandatory in the interpretation of clinical observations utilizing this marker as an endpoint.