Origin of the lag period in the phospholipase C cleavage of phospholipids in membranes. Concomitant vesicle aggregation and enzyme activation

When phospholipase C is added to a suspension of large unilamellar vesicles of egg phosphatidylcholine, maximal rates of hydrolysis occur only after a latency period. No lag period is seen when the substrate is in the form of small (sonicated) vesicles, or of short-chain phosphatidylcholine monomers. For a given vesicle concentration, the lag time may vary as a function of Ca2+, enzyme concentration, or temperature, but activation occurs at a fixed molar fraction of diacylglycerol produced. Lag times decrease gradually with vesicle size, and also with the amount of diacylglycerol present in the bilayers when it is mixed with phospholipid prior to enzyme addition. Parallel recordings of enzyme activity and suspension turbidity reveal that in all cases the latency period ends concomitantly with the start of a process of vesicle aggregation. Both the lag time and the amount of diacylglycerol formed before activation decrease with vesicle concentration, suggesting that enzyme activation is somehow related to vesicle aggregation. The latency period of phospholipase C may be explained in terms of a hypothesis according to which (a) full enzyme activity requires the presence of membrane surface irregularities or defects, (b) the diacylglycerol generated in the lag phase produces some kind of phase separation, with the formation of diacylglycerol-rich ''patches'' or domains, (c) vesicles aggregate through contacts between those patches, and (d) aggregation causes (and/or increases, and/or stabilizes) the surface inhomogeneities that allow fast enzyme activity. These data and suggestions may be relevant to the process of model membrane fusion promoted by phospholipase C.