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Pseudomonassp. CBB1 degraded caffeine via C-8 oxidation. Previously, a novel quinone-dependent caffeine dehydrogenase (Cdh) was shown to catalyze the oxidation of caffeine to 1,3,7-trimethyluric acid (TMU). Initial metabolite analysis using resting cells and partially purified extract of CBB1 identified transient accumulation 1,3,7-trimethyl-5-hydroxyisourate (TM-HIU), and 3,6,8-trimethylallantoin (TMA). TMA structure was confirmed; chiral analysis revealed that it was racemic. In contrast, a time-course reaction showed that one of the enantiomers of TMA accumulated nine times, and racemized in three hours. Based on this, it was proposed that TMU was converted to TM-HIU and enantiomeric TMA.
A 43-kDa NADH-dependent TMU mononxygenase (TmuM) was purified and shown to convert TMU to unstable TM-HIU. The enzyme belonged to a new family of FAD-dependent monooxygenases. The enzyme was specific for methyluric acid with no activity on uric acid. Homology model of TmuM revealed a larger, more hydrophobic active site compared to analogous uricase in the uric acid pathway.
Genes encoding heterotrimeric Cdh (cdhA,B,C) and TmuM (tmuM), were located on a 25.2-kb fragment in CBB1 genome. Gene cluster analysis relative to similar cluster in uric acid degrading organisms identified five more putative genes of the C-8 oxidation pathway, namely tmuH, tmuD, orf1, orf2, and orf3. First three genes were assigned encoding TM-HIU hydrolase (TM-HIU to TM-OHCU), TM-OHCU decarboxylase (TM-OHCU to stereospecific TMA (proposed S-(+)-TMA)), and trimethylallantoinase (stereospecific TMA to TMAA), respectively. Further, orf2 and orf3 are proposed to encode for YlbA and ArgE like hydrolase and deacetylase, which convert TMAA to glyoxylate, di- and monomethylurea. This is the first report of (a) TMA structure (b) TMU monooxygenase and TM-HIU (hydroxylation product of TMU), and (c) complete delineation of C-8 oxidation pathway by a combination of enzymology and cluster analysis.
Excessive consumption of caffeine in various forms has created a need for a rapid diagnostic test, esp. for nursing mothers and infants. Cdh was hypothesized to be suitable for this test. Sensitivity of the test was shown to be 1 ppm. A colorimetric test with partially purified Cdh and INT-dye was optimized to detect within a minute, caffeine in drugs, nursing mother's milk, and differentiate decaffeinated beverages.