30. ^<18>O標識アブシシン酸メチルエステルの1,1',4'位酸素官能基のEIMSによる識別

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Abscisic acid (ABA) possesses oxygen atoms at C-1, 1' and 4'. Elucidation of origin of these oxygen atoms may reveal enzymes catalyzing introduction of these oxygen atoms. Labeling experiment with ^<18>O_2 or H_2^<18>O is usually used to examine origin of oxygen. We found that EIMS analysis of ^<18>O-labeled ABA-Me could distinguish oxygen atoms at C-1, 1' and 4' in the course of labeling studies with ^<18>O_2 in ABA-producing fungi. ABA-Me (MW 278) gives the fragment ion containing 1-oxygen atoms at m/z 125. This ion of ABA-Me labeled at C-1 with one or two ^<18>O shifted to m/z 127 or 129, respectively, showing that incorporation of ^<18>O into C-1 can be confirmed by the fragment ion. ABA-Me shows a dehydrated ion at m/z 260. Sixty-five percent of the dehydrated ion is formed by elimination of the 1'-OH and 4- or 5-H, but the eliminating group giving the residual 35% of dehydrated ion has been still unknown. ABA methyl esters labeled with stable isotopes showed that the unknown dehydrated ion was derived from elimination of 4'-carbonyl oxygen with hydrogen atoms at the 1'-OH and at C-4 or -5 or -3' or -5' or -7'. Dehydration by loss of the carbonyl oxygen is an unusual fragmentation. This finding shows that ^<18>O-labeled positions between C-1' and C-4' can be distinguished by their ratios of relative intensity of dehydrated ions. [1'-^<18>O]ABA-Me gives dehydrated ions [M+2-H_2O]^+ at m/z 262 and [M+2-H_2^<18>O]^+ at m/z 260 in a 35:65 ratio, and [4'-^<18>O]ABA-Me in a 65:35 ratio. Methyl ester of phaseic acid (PA-Me, MW 294), the natural catabolite of ABA, gave the dehydrated ion (m/z 276) derived from loss of 1'-OH and 4'-O in a 27:73 ratio. This means that [1'-^<18>O] and [4'-^<18>O]PA-Mes can be distinguished by difference of relative intensities of the dehydrated ions also.
植物化学調節学会の論文
2004-10-13

30. ^<18>O標識アブシシン酸メチルエステルの1,1',4'位酸素官能基のEIMSによる識別

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