Analysis of microsomal cholesteryl ester hydrolases by radiation inactivation

Effects of high-energy electrons and gamma rays directly on protein molecules

High-energy electrons and gamma rays ionize molecules at random along their trajectories. In each event, chemical bonds are ruptured, releasing radiolytic products that diffuse away. A solution of macromolecules is mostly water whose principal radiation products are H(+) and OH(-). These can diffuse to and react with macromolecules; this indirect action of radiation is responsible for 99.9% of the damage to proteins. In frozen samples, the ionizations still occur randomly and water is still the principle molecular target, but diffusion of radiation products is limited to only a very small distance. At very low temperatures, essentially all the radiation damage to macromolecules is due to primary ionizations occurring directly in those molecules. Therefore, proteins in frozen solutions are only 10(-3) to 10(-4) as sensitive to radiation as in the liquid state. Every molecule that suffered a direct ionization is destroyed; the only surviving molecules are those that escaped ionization. The survival of frozen proteins after irradiation is a direct measure of the mass of the active structures and independent of the presence of other proteins.

Size of the catalytically active unit of rat hepatic carboxylester lipase in the presence and absence of bile salt

Carboxylester lipase (CEL) catalyzes the hydrolysis of cholesteryl esters, retinyl esters, and triacylglycerols. CEL monomer has a MW of approximately 70000. Hydrolysis of these esters is stimulated by millimolar trihydroxy bile salts such as cholate, that also induce aggregation. Liver cytosols from 12 rats were frozen and irradiated at -135 degrees C with high energy electrons. In several experiments, paired samples of cytosol were adjusted to 20 mM cholate before irradiation. All samples were assayed for CEL using cholesteryl oleate as substrate. In untreated cytosols, CEL activity surviving radiation exposure could be fit to a single exponential function, the slope of which yielded a target size of 91 +/- 18 kDa. In a subset of these cytosols irradiated in the presence of cholate the calculated target size was 100 +/- 19 kDa, a value indistinguishable from that obtained for untreated cytosols. Some samples were also assayed using retinyl palmitate and triolein as substrates. With retinyl palmitate the mean target sizes were 96 and 108 kDa in the absence and presence of cholate, respectively, approximately the same as those observed when using cholesteryl oleate. When triolein was used as substrate the target sizes in the absence of cholate were smaller than for the other two esters (67 +/- 18 kDa) and closer to the known monomer molecular weight, but again cholate had no significant effect on this size. The structure responsible for CEL activity contains no more than one 70000 MW monomer and the results show that cholate-induced oligomerization is not required for catalytic activity.