Fluorination at the 4 position alters the substrate behavior of l-glutamine and l-glutamate: Implications for positron emission tomography of neoplasias

Abstract Two 4-fluoro- l -glutamine diastereoisomers [(2 S ,4 R )-4-FGln, (2 S ,4 S )-4-FGln] were previously developed for positron emission tomography. Label uptake into two tumor cell types was greater with [ 18 F](2 S ,4 R )-4-FGln than with [ 18 F](2 S ,4 S )-4-FGln. In the present work we investigated the enzymology of two diastereoisomers of 4-FGln, two diastereoisomers of 4-fluoroglutamate (4-FGlu) (potential metabolites of the 4-FGln diastereoisomers) and another fluoro-derivative of l -glutamine [(2 S ,4 S )-4-(3-fluoropropyl)glutamine (FP-Gln)]. The two 4-FGlu diastereoisomers were found to be moderate-to-good substrates relative to l -glutamate of glutamate dehydrogenase, aspartate aminotransferase and alanine aminotransferase. Additionally, alanine aminotransferase was shown to catalyze an unusual γ-elimination reaction with both 4-FGlu diastereoisomers. Both 4-FGlu diastereoisomers were shown to be poor substrates, but strong inhibitors of glutamine synthetase. Both 4-FGln diastereoisomers were shown to be poor substrates compared to l -glutamine of glutamine transaminase L and α-aminoadipate aminotransferase. However, (2 S ,4 R )-4-FGln was found to be a poor substrate of glutamine transaminase K, whereas (2 S ,4 S )-4-FGln was shown to be an excellent substrate. By contrast, FP-Gln was found to be a poor substrate of all enzymes examined. Evidently, substitution of H in position 4 by F in l -glutamine/ l -glutamate has moderate-to-profound effects on enzyme-catalyzed reactions. The present results: 1) show that 4-FGln and 4-FGlu diastereoisomers may be useful for studying active site topology of glutamate- and glutamine-utilizing enzymes; 2) provide a framework for understanding possible metabolic transformations in tumors of 18 F-labeled (2 S ,4 R )-4-FGln, (2 S ,4 S )-4-FGln, (2 S ,4 R )-4-FGlu or (2 S ,4 S )-4-FGlu; and 3) show that [ 18 F]FP-Gln is likely to be much less metabolically active in vivo than are the [ 18 F]4-FGln diastereoisomers.