The aldo-keto reductase family 7 (AKR7) proteins are known to play an important role in the detoxication of reactive carbonyl containing compounds including aromatic and aliphatic aldehydes. The rat AKR7A1 was first identified as an inducible aflatoxin dialdehyde reductase, catalyzing the formation of the less toxic alcohol metabolites. AKR7A1 has also been shown to detoxify acrolein. In humans two related proteins, AKR7A2 and AKR7A3, have been identified. To date, the expression and catalytic activities of AKR7A2 and AKR7A3 have not been compared, in part, because their DNA and protein sequences are very similar. To study the tissue expression of the AKR7A2 and AKR7A3 we developed a Real-time PCR method using Taqman probes that are unique to the untranslated regions of each mRNA. Samples from 15 tissues and leukocytes were analyzed. AKR7A2 was detected in all samples and copy number/μg mRNA was about 1 log greater that those of AKR7A3. AKR7A3 was more than 2 logs lower in the lung, muscle, spleen, thymus, placenta and leukocytes. The highest amount of both mRNAs was in the kidney and liver. Analysis of 12 liver samples indicated that levels of AKR7A2 exceeded those of AKR7A3. Western blot analysis using protein standards from transfected COS-7 cells and low bis-acrylamide concentrations allowed separation and identification of these two proteins. Except for one sample, the level of AKR7A2 was greater than that of AKR7A3, with a ratio of 2-fold or less. Across samples, the range of expression varied by as much as 10-fold. Using purified His-tagged recombinant proteins, we showed that AKR7A2 and AKR7A3 had similar catalytic efficiencies for the model substrates 9,10-phenanthrenequinone, 2-carboxybenzaldehyde and succinic semialdehyde, yet AKR7A3 had lower apparent Kmvalues. The catalytic efficiency for the reduction of aflatoxin dialdehyde was much higher for AKR7A3, and the apparent Kmvalues determined for AKR7A2 exceeded 500 μM, indicating low toxicologic relevance for the reduction of aflatoxin dialdehyde. COS-7 cells, transfected with the cDNA for either AKR7A2 or AKR7A3, were used to examine the effects of these enzymes on the cytotoxicity of acrolein. Expression of either enzyme afforded protection from acrolein, displacing the EC50 for cytotoxicity from 50 μM (vector control) to 150 μM (cDNA). These studies indicate important specific and over-lapping roles of human AKR7 proteins in the metabolism of natural and xenobiotic compounds.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]