Biology Reference
In-Depth Information
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.
Search WWH ::
Custom Search