ZERO-ORDER INTERFACIAL ENZYMATIC DEGRADATION OF PHOSPHOLIPID TUBULES

The first study of enzymatic hydrolysis of phospholipid tubules is rep orted. Phosphatidylcholines with acyl chains containing diacetylene gr oups are known to form tubular microstructures in which the lipids are tightly packed and crystalline. These tubules can be used to probe th e role of microstructural form in the mechanics of interfacial enzymat ic degradation by such enzymes as phospholipase A(2) (PLA(2)). Hydroly sis by PLA(2) may occur most: rapidly in regions having the greatest n umber of bilayer packing defects, such as those that must be found at tubule ends. A microstructure that degrades primarily from its ends sh ould exhibit zero-order kinetics, because the area of the degrading tu bule end remains constant as the length of the microstructure decrease s. Free fatty acid concentration was measured to follow the generation of PLA(2) hydrolysis products in suspensions of diacetylenic phosphol ipid tubules, The kinetics of tubule hydrolysis were essentially zero- order until conversion was complete, as predicted. However, microscopy of partially hydrolyzed tubules revealed the formation of multiple di screte anionic product domains along the length of degrading tubules a s well as in insoluble reaction product microstructures. Furthermore, the rate of tubule hydrolysis was only moderately enhanced by increasi ng the number of tubule ends, which is consistent with the conclusion that tubule ends are not the only sites of hydrolysis. A model that re conciles the overall kinetics with the morphological evidence is propo sed.