Phospholipid metabolism in membranes of senescing bean cotyledons

Mechanisms underlying the depletion of phospholipid in senescingmembranes have been examined using microsomes isolated frombean cotyledons (Phaseolus vulgaris) at various stages of development.As the cotyledons age, microsomal phospholipid levels relativeto protein decrease by 93% indicating that phospholipids areselectively depleted from senescing membranes. This reflectsactive phospholipid catabolism, but can also be attributed toa reduction in phospholipid synthesis. Specifically, the activitiesof choline phospho-transferase and ethanolamine phosphotransferase,enzymes mediating the terminal step in the synthesis of phosphatidylcholineand phosphatidylethanolamine, respectively, decrease dramaticallyas the cotyledons senesce. Phosphatidylcholine and phosphatidylethanolaminecomprise over 70% of the total phospholipid in these membranes,and this pronounced decline in their synthesis with advancingsenescence will lead to phospholipid depletion. There is alsoa decrease with age in the activity of acyl-CoA synthetase,which generates acyl-CoA for use in phospholipid synthesis.Microsomal phospholipid deacylation-reacylation activity declinesas well as the cotyledons senesce, but this can be accountedfor in terms of decreased levels of phospholipid available forthe reaction. Thus the depletion of phospholipid in senescingmembranes can be attributed to active catabolism in the faceof declining synthesis. Key words: Phospholipid synthesis, senescence, microsomes, Phaseolus vulgaris