Mechanisms underlying the depletion of phospholipid in senescing membr
anes have been examined using microsomes isolated from bean cotyledons
(Phaseolus vulgaris) at various stages of development. As the cotyled
ons age, microsomal phospholipid levels relative to protein decrease b
y 93% indicating that phospholipids are selectively depleted from sene
scing membranes. This reflects active phospholipid catabolism, but can
also be attributed to a reduction in phospholipid synthesis. Specific
ally, the activities of choline phosphotransferase and ethanolamine ph
osphotransferase, enzymes mediating the terminal step in the synthesis
of phosphatidylcholine and phosphatidylethanolamine, respectively, de
crease dramatically as the cotyledons senesce. Phosphatidylcholine and
phosphatidylethanolamine comprise over 70% of the total phospholipid
in these membranes, and this pronounced decline in their synthesis wit
h advancing senescence will lead to phospholipid depletion. There is a
lso a decrease with age in the activity of acyl-CoA synthetase, which
generates acyl-CoA for use in phospholipid synthesis, Microsomal phosp
holipid deacylation-reacylation activity declines as well as the cotyl
edons senesce, but this can be accounted for in terms of decreased lev
ers of phospholipid available for the reaction. Thus the depletion of
phospholipid in senescing membranes can be attributed to active catabo
lism in the face of declining synthesis.