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detectable in most green plants during all stages of greening during the light phase
of the photoperiod (Rebeiz, unpublished).
Photoconversion of MV Pchlide
a
E to MV Chlide
a
E
Addition of two trans-hydrogens across the 7-8 position of the MV Pchlide
a
ester macrocycle would result in the conversion of MV Pchlide
a
ester to MV
Chl
a
. Several laboratories have reported such a reaction in higher plants
(Belanger and Rebeiz
1980a
,
b
;Lanceretal.
1976
; Liljenberg
1974
;Rebeiz
and Castelfranco
1973
), and lower plant (Kotzabasis and Senger
1989
;Sasaand
Sugahara
1976
). Since other researchers have not been able to detect the
photoconversion of Pchlide
a
ester, Rudiger and Schoch suggested that such
discrepancies may be due to age of seedlings or the very rapid esterification of
Chlide
a
to Chl
a
during the light treatment (Rudiger and Schoch
1991
). The
latter possibility is unlikely as the photoconversion of Pchlide
a
ester has been
also observed at temperatures of
15 to 2
C (Liljenberg
1974
;Rebeizand
Castelfranco
1973
). In our opinion, failure to observe the photoconversion of
Pchlide
a
EtoChl
a
stems from two considerations: (a) The photoconversion is
only partial and very small amounts of Chl
a
are formed, (b) detection of such
small amounts of Chl
a
depends a great deal on the sensitivity of the used
instrumentation. We have recently reexamined the photoconversion of Pchlide
a
E to Chlide
a
E in isolated cucumber etioplasts. Reaction products were
determined by HPLC coupled to high resolution spectrofluorometric detection.
It was possible to show that isolated etioplasts of barley and corn subjected to a
2.5 ms flash of light at room temperature followed by immediate precipitation
with ammoniacal acetone at various temperatures resulted in the detection of
several Chlide a esters (Fig.
11.8B
). However, illumination of frozen etioplasts at
18 C did not photoreduce the Pchlide
a
E pool (Adra
1998
). These results
confirmed the partial photoconvertibility of Pchlide
a
E at room temperatures, but
raised the possibility that the enzyme responsible for (photo)reduction of Pchlide
a
E was much more sensitive to low temperatures than conventional Pchlide
a
oxidoreductases (Fig.
11.7
).
Light-Independent Conversion of MV Pchlide
a
E to MV Chl
a
E
On the basis of spectrophotometric and spectrofluorometric analysis, it is presently
assumed that the fully esterified tetrapyrrole pool of etiolated tissues consists
exclusively of Pchlide
a
E. Recently, it was conjectured that should small amounts
of other fully esterified tetrapyrroles be present, their detection would be obscured
by the presence of the much larger amounts of Pchlide
a
E. To test this hypothesis,
HPLC analysis of etiolated tissues extracts followed by on line spectrofluorometric
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