The Isolation and Identification of 6-Hydroxycyclohepta-1,4-Dione As a Novel Intermediate in the Bacterial Degradation of Atropine Barbara A
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115 Biochem. J. (1993) 293, 115-118 (Printed in Great Britain) 115 The isolation and identification of 6-hydroxycyclohepta-1,4-dione as a novel intermediate in the bacterial degradation of atropine Barbara A. BARTHOLOMEW, Michael J. SMITH, Marianne T. LONG, Paul J. DARCY, Peter W. TRUDGILL and David J. HOPPER* Department of Biochemistry, University of Wales, Aberystwyth, Dyfed SY23 3DD, Wales, U.K. Growth of Pseudomonas AT3 on the alkaloid atropine as its sole cyclohepta-1,4-dione. This novel compound is proposed as an source of carbon and nitrogen is nitrogen-limited and proceeds intermediate in tropine metabolism. It served as a growth by degradation of the tropic acid part of the molecule, with the substrate for the organism and was also the substrate for an metabolism of the tropine being limited to the point of release of NADI-linked dehydrogenase present in cell extracts. The enzyme its nitrogen. A nitrogen-free compound accumulated in the was induced during the tropine phase of diauxic growth on growth medium and was isolated and identified as 6-hydroxy- atropine or during growth on tropine alone. INTRODUCTION conditions, there also has to be partial metabolism of the tropine to provide nitrogen for growth. This results in the accumulation Atropine is one of the tropane alkaloids that occurs in plants of of a nitrogen-free product from tropine, and the present paper the family Solanaceae, including Atropa belladonna (deadly deals with the isolation and identification of this compound with nightshade) (Dalton, 1979) and was first isolated by Brandes some evidence that it is an intermediate in tropine metabolism. (1832). The molecule contains two cyclic structures, the alicyclic nitrogen-containing alcohol tropine and the aromatic tropic MATERIALS AND METHODS acid, joined together by an ester linkage (Figure 1). Its degrad- ation by micro-organisms has been reported by several groups Maintenance and growth of organism Niemeret Kackowski and (Kackowski, 1959; al., 1959; Mazejko- The organism was maintained and grown as described by Long Toczko, 1960; Berends et al., 1967; Rorsch et al., 1971), and in et al. (1993). Medium contained 1 g ofgrowth substrate/l. Growth those cases where the mechanism has been studied, the initial was monitored by measuring attenuance (D) at 580 nm. step is the hydrolysis of the ester bond to give the two separate cyclic components. The tropic acid moiety has been reported to be metabolized via phenylacetic acid (Stevens and Rorsch, 1971), Preparation of cell extracts but much less is known about the fate of the tropine molecule, or Cell paste was resuspended in an equal volume of 42 mM indeed about the metabolism of complex N-heterocycles in potassium/sodium phosphate buffer, pH 7.1, and cells were general. This is partly because suitable compounds for testing as disrupted by ultrasonic disintegration using a Soniprep 150 putative intermediates or for use as analytical standards are not (MSE, Crawley, Sussex, U.K.) sonifier at an amplitude of 14 ,m readily available and this has hindered studies of their degrad- with four 30 s bursts interspersed with cooling on ice. The ation. However, oxidation of the alcohol group of tropine by an disrupted suspension was then centrifuged at 28000 for 30 min NAD+-linked dehydrogenase in Corynebacterium belladonna to at 4 °C and the supernatant taken as crude extract. give tropinone has been reported by Niemer and Bucherer (1961). They also reported the isolation of methylamine and small amounts of tropinic acid (Figure 1) from culture medium Partial purfflcation of 6-hydroxycyclohepta-1,4-dione and suggested metabolism of the tropinone through tropinic acid dehydrogenase to 2,6-dioxopimelic acid. This implies cleavage of the alicyclic A portion ofcrude extract containing 5 mg ofprotein was loaded ring of tropine at the carbon bearing the hydroxy group before on to a column of 0.5 ml of Reactive Red 120-agarose (Sigma, release of nitrogen and plausible biochemical mechanisms to Poole, Dorset, U.K.) in 21 mM potassium/sodium phosphate accomplish this can be postulated. However, the evidence was buffer, pH 7.1, contained in a Pasteur pipette. The column was not conclusive. There has been little progress in the elucidation washed with 5 ml ofbuffer and then enzyme was eluted with 2 ml of the mechanism of tropine degradation since that time, of buffer containing 5 mM NADI. This gave a 10.7-fold purifica- reflecting in part some of the difficulties mentioned above. tion with 36 % recovery of activity. In this study an organism capable of growth on atropine as its sole carbon or and source carbon nitrogen has been isolated Product from dehydrogenase from soil and identified as a species of Pseudomonas. It, too, 6-hydroxycyclohepta-1,4-dione reaction initiates degradation by an esterase-catalysed cleavage of the substrate and then preferentially utilizes the aromatic fragment A reaction mixture containing, in 30 ml of 0.1 M sodium pyro- (Long et al., 1993). This fragment (tropic acid) is also prefer- phosphate buffer, pH 8.8, NADI (90,umol), 6-hydroxy- entially utilized when the organism is grown on atropine as its cyclohepta-1,4-dione (4.9 mg) and partially purified enzyme sole source of both carbon and nitrogen but, under these (0.23 mg) was incubated at 30 °C for 4 h. The mixture was then Abbreviation used: 2,4-DNPH, 2,4-dinotrophenylhydrazine. * To whom correspondence should be addressed. 116 B. A. Bartholomew and others CH3 G.c-m.s. Ns G.c.-m.s. was performed on a Hewlett-Packard 5890 instrument with a 5971 mass-selective detector. An HP-5 (cross-linked 5 % phenylmethylsilicone) column (25 m x 0.2 mm x 0.33 ,tm film) with helium as the carrier gas, and a temperature programme of 11 OH 4 min at 70 °C rising at 10 °C/min to 275 'C, were used. 0 II N.m.r. spectra N.m.r. spectra were performed by Dr. 0. W. Howart at the CH2CO2H SERC n.m.r. Service, The University ofWarwick, using a Bruker H WH 400 spectrometer with the sample dissolved in [2H]- HO2C C N-CH3 chloroform. CH20H CO2H I.r. spectra III IV The i.r. spectrum of a 1 % KBr mull of the product was obtained using a Perkin-Elmer 1310 instrument. Figure 1 Formulae of (I) atropine, (11) tropine, (111) tropic acid and (IV) tmpinic acid Microanalysis Microanalysis for C and H was performed by M. Hart, Micro- analytical Laboratory, Department of Chemistry, University of acidified to pH 1-2 with 5 M HCI and extracted nine times with Manchester, Manchester, U.K. 30 ml of diethyl ether. The pooled extracts were dried over anhydrous Na2SO4 and the diethyl ether removed on a rotary evaporator to leave a small amount of an oil as product. RESULTS AND DISCUSSION Accumulaton of Intermediate 6-Hydroxycyclohepta-1,4-dione dehydrogenase assay Growth of Pseudomonas AT3 on atropine as its sole source of The hydroxycyclohepta- 1 ,4-dione dehydrogenase was assayed carbon is diauxic, with hydrolysis of the molecule and utilization spectrophotometrically by monitoring the reduction of NADI at of the tropic acid portion in the first phase of growth followed, 340 nm at 30 °C in a 1-cm-pathlength cuvette containing in after a lag, by growth on the tropine (Long et al., 1993). Assay 3.0 ml of 0.1 M sodium pyrophosphate buffer, pH 8.8, 2 ,umol of of the growth medium showed the transient appearance of 2,4- NADI, 0.2 ,umol of substrate and enzyme. A unit of activity was DNPH-reactive material, indicative of an aldehyde or ketone, that causing the reduction of 1 ,umol of NADI/min. during the tropine growth phase. Similar transient formation of small amounts of such material was seen when tropine alone was used as the sole carbon, or carbon and nitrogen, source(s). Under Proteln assays conditions where atropine was the sole carbon and nitrogen Protein was assayed by the method of Lowry et al. (1951), with source, the culture was nitrogen-limited and the extent of growth BSA as standard. was reduced to the equivalent of that for the tropic acid phase. Under these conditions there was accumulation of much larger amounts of 2,4-DNPH-positive material which declined slowly Assay for ketones on prolonged incubation of the culture (Figure 2), suggesting Ketones were detected by their reaction with 2,4-dinitrophenyl- only limited metabolism of the tropine, mainly to the point of hydrazine (2,4-DNPH). The reagent contained 0.1 % (w/v) 2,4- removal of the nitrogen required for growth. To accumulate DNPH in 2 M HCl. A semiquantitative assay for ketones in sufficient of this material for identification a 500 ml culture was culture medium was performed, after removal of cells from a monitored for 2,4-DNPH-positive material during growth on sample of culture by centrifuging, by incubation of a portion of 500 mg of atropine as the sole carbon and nitrogen source. At the supernatant, diluted to 0.5 ml, with 0.5 ml of 2,4-DNPH maximum metabolite accumulation the cells were removed and reagent at 30 °C for 30 min. After addition of 2 ml of 1 M NaOH the medium was extracted continuously with diethyl ether. The and incubation for 5 min at room temperature, the absorbance diethyl ether extract was dried over anhydrous Na2S04 and of the mixture was measured at 427 nm. evaporated to dryness to yield a pale yellow oil. This was purified by flash chromatography on silica gel with ethanol as solvent. The solvent was removed by evaporation and the product Chromatography crystallized from diethyl ether, typically giving 100 mg of white Flash chromatography was performed on a 15 cm x 2 cm column crystals, m.p.