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Fig. 13.2 Monovinyl
(MV) and Divinyl
(DV) Chlorophyllide b
Metabolism of MV Chlide b
MV Chlide b was simultaneously detected in greening (Duggan and Rebeiz. 1981 ,
1982 ) and green higher plant tissues (Aronoff 1981 ). It was proposed as a logical
immediate precursor of MV Chl b . Subsequently the conversion of exogenous MV
Chlide b to MV Chl b in etiolated oat was reported (Benz and Rudiger 1981 ).
The pool of MV Chlide b exhibited the spectrofluorometric properties of MV
Chl b in diethyl ether at 298 and 77 K, but had the chromatographic mobility and
solubility of a monocarboxylic phorbin. The presence of a free carboxylic group
and a formyl group was demonstrated by methylation with diazomethane and
conversion to a Chlide b oxime upon reaction with hydroxylamine (Duggan and
Rebeiz 1982 ). The concentration of Chlide b in green tissues was in the same range
as that of MV Pchlide a , and MV Chlide a . It was estimated that less than 15 % of
the Chlide b pool could have arisen by hydrolysis of phytol at position 7 of the
macrocycle via chlorophyllase activity in vitro. This was confirmed by the extent of
hydrolysis of 14 C-labeled MV Chl b added to green tissues just before pigment
extraction (Duggan and Rebeiz 1982 ).
The source of oxygen of the formyl group at position 3 of the macrocycle has
been investigated by Porra et al. ( 1993 , 1994 ). Mass spectra of [7-hydroxymethyl]-
Chl b extracted from leaves greened in the presence of either 18 O 2 or H 2 18 O 2
revealed that 18 O was incorporated only from molecular oxygen into the 3-formyl
group of Chl b . The high enrichment using 18 O 2 , and the absence of labeling by
H 2 18 O 2 , suggested that molecular oxygen is the sole precursor of the 3-formyl
oxygen of Chl(ide) b in greening maize leaves. This in turn suggested that a
mono-oxygenase is involved in the oxidation of the methyl group to a formyl.
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