Importance of sulfhydryl group for rabbit gastric lipase activity.

We have shown recently that rabbit gastric lipase (RGL) purified from gastric tissue presents catalytic properties comparable with those of human gastric lipase (HGL). We report here that only one sulfhydryl group was modified per molecule of native RGL after incubation at pH 8.0 with 5,5'-dithiobis(2-nitrobenzoic acid) (NbS2) for 4 h or 4,4'-dithiopyridine (4-PDS) for 60 min. With both reagents, a direct correlation was found between the modification of one sulfhydryl group per enzyme molecule and loss of RGL activity. Incubation of RGL with the new hydrophobic sulfhydryl reagent, dodecyldithio-5-(2-nitrobenzoic acid) (C12-NbS), at 30-fold molar excess, at pH 3.0, 5.0 and 8.0, induced immediate and complete inactivation of RGL. Unlike NbS2 and 4-PDS, C12-NbS almost instantaneously stopped the course of tributyrin hydrolysis by RGL, in contrast to porcine pancreatic lipase (PPL). RGL can be included with HGL in the group of sulfhydryl enzymes.