Major differences exist in the function and tissue-specific expression of human aflatoxin B1 aldehyde reductase and the principal human aldo-keto reductase AKR1 family members

Abstract

Complementary DNA clones encoding human aflatoxin B(1) aldehyde reductase (AKR7A2), aldehyde reductase (AKR1A1), aldose reductase (AKR1B1), dihydrodiol dehydrogenase 1 (AKR1C1) and chlordecone reductase (AKR1C4) have been expressed in Escherichia coli. These members of the aldo-keto reductase (AKR) superfamily have been purified from E. coli as recombinant proteins. The recently identified AKR7A2 was shown to differ from the AKR1 isoenzymes in being able to catalyse the reduction of 2-carboxybenzaldehyde. Also, AKR7A2 was found to exhibit a narrow substrate specificity, with activity being restricted to succinic semialdehyde (SSA), 2-nitrobenzaldehyde, pyridine-2-aldehyde, isatin, 1,2-naphthoquinone (1,2-NQ) and 9,10-phenanthrenequinone. In contrast, AKR1A1 reduces a broad spectrum of carbonyl-containing compounds, displaying highest specific activity for SSA, 4-carboxybenzaldehyde, 4-nitrobenzaldehyde, pyridine-3-aldehyde, pyridine-4-aldehyde, 4-hydroxynonenal, phenylglyoxal, methylglyoxal, 2,3-hexanedione, 1, 2-NQ, 16-ketoestrone and d-glucuronic acid. Comparison between the kinetic properties of AKR7A2 and AKR1A1 showed that both recombinant enzymes exhibited roughly similar k(cat)/K(m) values for SSA, 1,2-NQ and 16-ketoestrone. Many of the compounds which are substrates for AKR1A1 also serve as substrates for AKR1B1, though the latter enzyme was shown to display a specific activity significantly less than that of AKR1A1 for most of the aromatic and aliphatic aldehydes studied. Neither AKR1C1 nor AKR1C4 was found to possess high reductase activity towards aliphatic aldehydes, aromatic aldehydes, aldoses or dicarbonyls. However, unlike AKR1A1 and AKR1B1, both AKR1C1 and AKR1C4 were able to catalyse the oxidation of 1-acenaphthenol and, in addition, AKR1C4 could oxidize di- and tri-hydroxylated bile acids. Specific antibodies raised against AKR7A2, AKR1A1, AKR1B1, AKR1C1 and AKR1C4 have been used to show the presence of all of the reductases in human hepatic cytosol; the levels of AKR1B1 and AKR1C1 were markedly elevated in livers with alcohol-associated injury, and indeed AKR1B1 was only detectable in livers with evidence of alcoholic liver disease. Western blotting of extracts from brain, heart, kidney, liver, lung, prostate, skeletal muscle, small intestine, spleen and testis showed that AKR7A2 is present in all of the organs examined, and AKR1B1 is similarly widely distributed in human tissues. These experiments revealed however, that the expression of AKR1A1 is restricted primarily to brain, kidney, liver and small intestine. The AKR1C family members proved not to be as widely expressed as the other reductases, with AKR1C1 being observed in only kidney, liver and testis, and AKR1C4 being found in liver alone. As human kidney is a rich source of AKR, the isoenzymes in this organ have been studied further. Anion-exchange chromatography of human renal cytosol on Q-Sepharose allowed resolution of AKR1A1, AKR1B1, AKR1C1 and AKR7A2, as identified by substrate specificity and Western blotting. Immunohistochemistry...

Detection and identification of tumor-associated protein variants in human hepatocellular carcinomas

“…AR was described as elevated in livers with alcohol-associated injury and disease. 47 This agrees with our findings, that 2 HCCs originating from alcoholics show not only a remarkable increase of AR but also high hARLP amounts. In these cases, toxic acetaldehyde, a degradation metabolite of alcohol, might have played a role in the induction of these human HCCs.…”

Activation of osmolyte pathways in inflammatory myopathy and Duchenne muscular dystrophy points to osmoregulation as a contributing pathogenic mechanism

“…14 Of the three NFAT5 targets involved in osmolyte accumulation we tested for this study, we found only AR to be present in substantial amounts in normal muscle, which confirms an earlier report 14 and points to a physiological role in muscle homeostasis. 15 Levels of TauT (The Human Protein Atlas: http://www.proteinatlas. org) and SMIT mRNA in the human skeletal muscle are low 16,17 and we could not detect the corresponding protein in normal muscle samples.…”

Characterisation of a novel mouse liver aldo‐keto reductase AKR7A5

“…testis and, importantly, brain [33]. The narrow tissue distribution of the rat enzyme suggested that it has a specialised tissue speci¢c role, possibly in detoxication.…”

“…It is possible that the inducibility of AKR7A1 is a feature unique to the rat. Neither of the human enzymes AKR7A2 and AKR7A3 has been shown to be inducible in the liver, though there does appear to be some interindividual variation in expression levels and activities [21,33].…”