Analysis of microsomal cholesteryl ester hydrolases by radiation inactivation

Radiation inactivation by high energy electrons, a method for determining the size of a protein without prior purification, was used to study the acid and neutral cholesteryl ester hydrolase (CEH) activities of rat liver microsomes. The same preparations were also assayed for the microsomal, "nonspecific" carboxylesterases using o-nitrophenyl acetate as substrate. Non-specific esterase activity surviving radiation could be fit to a single exponential function, the slope of which yielded a target size of 47 ± 5 kDa (mean ± S.D., n = 7). Surviving CEH activity assayed at pH 5 could also be fit to a single exponential that yielded a target size of 71 ± 14 kDa (n = 5). In contrast, the surviving CEH activity assayed at pH 7 was more complex. The data from six experiments were described as the sum of two exponentials, indicating that most of the activity is due to an entity that is three to four times larger and a minor amount to one that is half the size of the pH 5 enzyme. The results are consistent with the suggestion that the acid and neutral microsomal CEH activities are due to distinct enzymes, which are not the "nonspecific" carboxylesterases. Their sizes also differ from those previously determined for lysosomal acid lipase and other lipases in the liver.