HENYL)-8-HYDROXY-6-OXO-3-OXABICYLO[3.3.0]-7-OCTENE - UNUSUAL PRODUCT OF THE SOYBEAN LIPOXYGENASE-CATALYZED OXYGENATION OF CURCUMIN

Racemic (1SR,2 ,4SR,5SR)-2-[(4'-hydroxy-3'-methoxy)-phenoxy]-4-(4 ''-3 nyl)-8-hydroxyl-6-oxo-3-oxabicyclo[3.3.0]-7-octene (2, C21H20O8) was isolated as major product of soybean lipoxygenase action on curcumin ( 1, C21H20O6). The structure of 2 was elucidated by HPLC-APCI-MS and ta ndem MS, H-1, C-13, DEFT, H,H-COSY, H,C-HMQC, H,C-HMBC and phase sensi tive 2D NOESY NMR techniques. For kinetic studies the rate of substrat e degradation was followed spectrophotometrically at 430 nm, and the r ate of oxygen consumption was measured polarographically. As evaluated by both methods, K-m for 1 was about four times higher than that obta ined for linoleic acid (as the best substrate for soybean lipoxygenase ); V-max was reduced five-fold. Lipoxygenase-mediated oxygenation of 1 was confirmed by the following criteria: (i) curcumin did not react w ith inactivated lipoxygenase; (ii) the enzymatic reaction was strongly inhibited by inhibitors such as BHA, deferoxamine and HgCl2; (iii) ox ygen consumption (measured polarographically) and curcumin degradation (measured photometrically) were shown to occur simultaneously at a ra tio of 0.8 to 1, suggesting insertion of oxygen into I by lipoxygenase ; (iv) molecular mass estimation by APCI-MS showed a shift of 32 in mo lecular mass from 1 (M-r 368) to 2 (M-r 400) being equivalent to an in sertion of dioxygen. Curcumin meets none of the common features for li poxygenase substrates and, therefore, may represent a new type of subs trates for this enzyme. (C) 1998 Elsevier Science B.V. All rights rese rved.