Isatins Inhibit N⁵-CAIR Synthetase by a Substrate Depletion Mechanism

Abstract

The continued rise of antibiotic-resistant infections coupled with the limited pipeline of new antimicrobials highlights the pressing need for the development of new antibacterial agents. One potential pathway for new agents is de novo purine biosynthesis as studies have shown that bacteria and lower eukaryotes synthesize purines differently than humans. Microorganisms utilize two enzymes, N 5 -CAIR synthetase and N 5 -CAIR mutase, to convert 5-aminoimidazole ribonucleotide (AIR) into 4-carboxy-5-aminoimidazole ribonucleotide (CAIR) through the intermediate N 5 -carboxy-5-aminoimidazole ribonucleotide (N 5 -CAIR). In contrast, vertebrates directly convert AIR to CAIR via the enzyme AIR carboxylase. A high-throughput screen against N 5 -CAIR synthetase identified a group of compounds with a 2,3-indolinedione (isatin) core that inhibited the enzyme. While initial studies suggested that isatins inhibited the enzyme by a noncompetitive mechanism, here we show that isatins inhibit N 5 -CAIR synthetase by a substrate depletion mechanism. Unexpectedly, we found that isatin reacts rapidly and reversibly with the substrate AIR. The rate of the reaction is dependent upon the substituents on the phenyl moiety of isatin, with 5-and 7-bromoisatin being faster than 4-bromoisatin. These studies suggest that care should be taken when exploring isatin compounds because the biological activity could be a result of their reactivity.

Applications of Purine Ring Opening in the Synthesis of Imidazole, Pyrimidine, and New Purine Derivatives

“… [51] The mechanism can be evidently explained by diamine attack at the C ‐2 position of hypoxanthine 58 , cyclization of 59 to the tetrahydroimidazole adduct 60 , followed by elimination of dihydroimidazole and DNP to yield the ring opening product 61 (AICA) [65,66] . In such way, DNCB together with EDA provides an easy access to different AICA derivatives: tris‐ O ‐TBS 62 a , [67] 5‐deoxy‐5‐fluoro 62 b , [68] 5‐ O ‐TBS‐2,3‐isopropylidene 62 c , [69] 13 C 5 labelled 62 d , [70] 2‐deoxy 62 e [51] and 62 f , [71] thio‐ 62 g and seleno‐ 62 h [72] riboside and the open‐chain ribose 62 i [56] analogues from simple inosines.…”

Structure–function characterization of an aldo–keto reductase involved in detoxification of the mycotoxin, deoxynivalenol

“…Amongst the ketone substrates, DepB Rleg possesses the highest specificity constant for 9,10-PQ while comparable specificity constants were observed for the other diketone, isatin and 3-keto-DON. 9,10-PQ is a xenobiotic air pollutant that triggers the generation of reactive oxygen species (ROS) in vivo 12 while isatin is an endogenous indole derived from tryptophan or phenylalanine metabolism by gut bacteria and has cytotoxic effects on microbes and various human cancer cell lines 55 . Overall, higher specificity constants were observed for ketones, versus aldehyde substrates.…”

Structure-Function characterization of an aldo-keto reductase involved in detoxification of the mycotoxin, deoxynivalenol

“…It had a lower speci city constant for isatin, another diketone, owing to the high K m value for this substrate. 9,10-PQ is a xenobiotic air pollutant that triggers the generation of reactive oxygen species (ROS) in-vivo 12 while isatin is an endogenous indole derived from tryptophan or phenylalanine metabolism by gut bacteria and has cytotoxic effects on microbes and various human cancer cell lines 55 . Overall, higher speci city constants were observed for ketones, versus aldehyde substrates.…”