Development of Analytical and Preparatory Techniques - Chlorophyll Biosynthesis and Technological Applications - page 126

where
Q B1 ¼ amount of radiolabel incorporated into compound B by the end of time

interval t 0 -t 1 .
Ya 1 ¼

specific radioactivity of compound A by the end of time interval t 0 -t 1 .

Δ B 1 ¼

amount of B synthesized by the end of time interval t 0 -t 1

3.9.3 Evaluation of the Contribution of “A” to the Formation
of “B” in Pathway II

If the comparison of calculated and experimental results indicates that compound
B is not formed from precursor P via compound A (Fig. 3.1a ), then the question
arises as to whether B is formed via pathway II or via pathway III. Furthermore, if
B is found to be formed via pathway II, then the contribution of A to the formation
of B needs to be assessed.
The determination of whether B is formed via pathway II or pathway III, can
be achieved from in vitro investigations. In other words, if cell-free systems are
available (see Chap. 4 ), for the particular pathway under consideration, then
the presence or absence of precursor-product relationship between A and B can be
readily demonstrated from conventional in vitro precursor-product conversions. For
example, in investigating the precursor-product relationship between ALA, DV
Pchlide and MV Pchlide in barley, (Tripathy and Rebeiz 1988 ) first determined that
MV Pchlide was not formed from ALA via pathway I. Then they showed that MV
Pchlide was formed from both ALA and DV Pchlide via pathway II and not via
pathway III by demonstrating the conversion of DV Pchlide to MV Pchlide in vitro.
If pathway II is found to be operational as in Tripathy and Rebeiz ( 1988 ), then
the contribution of A to the formation of B can be readily assessed from the
difference between the theoretical and experimental

l4 C-incorporations of P into

B , in vivo.
By assuming that the differences between the theoretical and experimental
in vivo 14 C-incorporation of P into B are due to the contribution of A to the
formation of B , then the maximum possible percent conversion of A to B can be
taken as:

% conversion ¼ 100 ð j Exp Q Bx j= Exp Þ 100 (3.104)

Where:
% conversion ¼ maximum possible percent conversion of A to B during any

time interval x .
Exp actual 14 C-incorporation into B by the end of time interval x, which is

determined experimentally.
Q BX ¼

¼

theoretical 14 C-incorporation into B by the end of time interval x which is
calculated with Eq. ( 3.101 ).