PR-104, currently in phase II clinical trials, is a phosphate ester pre-prodrug which is converted in vivo to its cognate alcohol, PR-104A, a prodrug designed to exploit tumor hypoxia. Bioactivation occurs via one-electron reduction to DNA crosslinking metabolites in the absence of oxygen. However, certain tumor cell lines activate PR-104A in the presence of oxygen, suggesting the existence of an aerobic nitroreductase. Microarray analysis identified a cluster of five aldo-keto reductase (AKR) family members whose expressions correlated with aerobic metabolism of PR-104A. Plasmid-based expression of candidate genes identified aldo-keto reductase 1C3 as a novel nitroreductase. AKR1C3 protein was detected by Western blot in 7 of 23 cell lines and correlated with oxic PR-104A metabolism, an activity which could be partially suppressed by Nrf2 RNAi knockdown (or induced by Keap1 RNAi), indicating regulation by the ARE pathway. AKR1C3 was unable to sensitize cells to 10 other bioreductive prodrugs and was associated with single-agent PR-104 activity across a panel of 9 human tumor xenograft models. Overexpression in two AKR1C3-negative tumor xenograft models strongly enhanced PR-104 antitumor activity. A population level survey of AKR1C3 expression in 2,490 individual cases across 19 cancer types using tissue microarrays revealed marked upregulation of AKR1C3 in a subset including hepatocellular, bladder, renal, gastric, and non-small cell lung carcinoma. A survey of normal tissue AKR1C3 expression suggests the potential for tumor-selective PR-104A activation by this mechanism. These findings have significant implications for the clinical development of PR-104. Cancer Res; 70(4); 1573-84.

Glyceryl-ether monooxygenase (1-alkyl-sn-glycerol,tetrahydropteridine: oxygen oxidoreductase, EC 1.14.16.5) catalyzes the oxidative cleavage of 1-O-alkyl glycerol or glycol derivatives to a long-chain aldehyde and the glycerol or glycol derivative. The specificity for tetrahydropterins of a similar, perhaps identical, enzyme that cleaves O-hexadecyl ethylene glycol in rat liver microsomes was examined with the use of an assay based on [1-3H]ethylene glycol formation from 2-hexadecyloxy [1-3H]ethan-1-ol. Several tetrahydropterin derivatives are effective electron donors for this reaction, and 2,4,5-triamino-6-hydroxypyrimidine is somewhat effective, but NADH, NADPH, ascorbate, reduced dichlorophenolindophenol and glutathione are inactive. Tetrahydropterin derivatives differ from each other in apparent Km and apparent Vmax. The order of increasing apparent Km values is tetrahydropterin approximately 6-methyltetrahydropterin approximately tetrahydrobiopterin less than 6.7-dimethyltetrahydropterin less than tetrahydrofolate. The order of increasing apparent Vmax values is tetrahydrofolate approximately tetrahydropterin less than 6-methyltetrahydropterin approximately tetrahydrobiopterin approximately 6,7-dimethyltetrahydropterin. Results obtained with the use of a spectrophotometric assay, in which tetrahydropterin oxidation is coupled to NADH oxidation by dihydropteridine reductase (NAD(P)H: 6,7-dihydropteridine oxidoreductase, EC 1.6.99.7), indicated that the ratio of 6,7-dimethyltetrahydropterin or 6-methyltetrahydropterin oxidized to ether lipid degraded is about 1.1 to 1.3. Unlike cytochrome P-450-dependent hydroxylases, this alkyl glycol-ether monooxygenase is not inhibited by carbon monoxide. 1-O-hexadecyl-rac-glycerol (chimyl alcohol) competitively inhibits the oxidation of the glycol ether indicating that the same enzyme probably catalyzes the oxidation of both O-alkyl glycol and 1-O-alkyl glycerol.

Tyrosine hydroxylase (TH, EC 1.14.16.2) from beef brain striata was purified 23-fold from an extract of an acetone powder. If this enzyme preparation is treated with a cyclic AMP[-dependent protein phosphorylation system, there is a change in the pH dependence of the enzyme activity. The pH optimum at saturating tetrahydrobiopterin (BH4) concentration is shifted from below pH 6 to about pH 6.7. At pH 7, activation is expressed mainly as an increase in Vmax, whereas at pH 6, activation is expressed mainly as a decrease in Km for the pterin cofactor. Further, even with the control enzyme the Km for pterin cofactor declines precipitously as the pH is increased from 6 toward neutrality. Similar data were obtained with G-25 Sephadex-treated rat striatal TH. Experiments in which rat striatal synaptosomes were used demonstrated that the in situ activation of TH by phosphorylating conditions is expressed primarily as an increase in the maximum rate of dopamine synthesis. These results indicate that changes in TH activity caused by cyclic AMP-dependent protein phosphorylation will depend to a large extent on the pH of the TH environment.

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Abstract— The tetrahydrofolate‐dependent serine hydroxymethyl transferase (l‐serine: tetrahydrofolate 10‐hydroxymethyl transferase, EC 2.1.2.1) reaction in rat or human brain homogenates incubated aerobically is dependent on added reducing agents for full activity in order to protect the readily oxidized substrate, tetrahydrofolate. In this role, 0.1 mm‐NADH is as affective as 10mm‐2‐mercaptoethanol and it can be shown that the NADH prevents destruction of tetrahydrofolate incubated with brain homogenates. If the dihydropteridine reductase (NADPH:6,7‐dihydropteridine oxidoreductase, EC 1.6.99.7) activity of the brain homogenate is inhibited by a specific antiserum, NADH, but not 2‐mercaptoeth‐anol, is no longer effective. Furthermore, an homogenate of a brain biopsy from a human lacking dihydropteridine reductase requires added dihydropteridine reductase for maximal stimulation by NADH of the serine hydroxymethyl transferase reaction. We conclude that dihydropteridine reductase mediates the NADH stimulation and can play a role in preserving tetrahydrofolate from oxidation. The rinding of greatly reduced folate levels in the brain biopsy from the human lacking dihydropteridine reductase supports this postulated role of dihydropteridine reductase in folate metabolism.

Rates of phosphatidate synthesis from dihydroxyacetone phosphate via acyl dihydroxyacetone phosphate or glycerol phosphate are compared in homogenates of 13 tissues, most of which are deficient in glycerol phosphate dehydrogenase (EC 1.1 .1.8). In all tissues examined, dihydroxyacetone phosphate entered phosphatidate more rapidly via acyl dihydroxyacetone phosphate than via glycerol phosphate. Tissues with a relatively low rate of phosphatidate synthesis via glycerol phosphate, showed no compensating increase in the rate of synthesis via acyl dihydroxyacetone phosphate. The rates at which tissue homogenates synthesize phosphatidate from dihydroxyacetone phosphate via glycerol phosphate increase as glycerol phosphate dehydrogenase increases. Both glycerol phosphate dehydrogenase and glycerol phosphate: acyl CoA acyltransferase (EC 2.3.1.15) are more active than dihydroxyacetone phosphate : acyl CoA acyltransferase (EC 2.3.1.42). Thus, all the tissue homogenates possessed an apparently greater capability to synthesize phosphatidate via glycerol phosphate than via acyl dihydroxyacetone phosphate, but did not express this potential. This result is discussed in relation to in vivo substrate limitations.

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