The inhibitory profiles of hog pancreatic and human rheumatoid synovial cell phospholipases A₂

Conclusions

The protective effect of Harmon's and Rosenthal's agents, and of the Upjohn inhibitors [4] is most simply ascribed to their shielding of His-48 by reversible binding at the enzyme's active site. However, protection by an allosteric mechanism cannot be excluded for any of these compounds. The lack of protection shown by other reversible inhibitors suggests that these agents do not bind to the enzyme active site. Some of these agents e.g. mepacrine, may inhibit PLA₂ by interacting with its membrane substrate, so perturbing its architecture and electrostatic properties, and thus interfering with enzyme approach and/or penetration [5]. Further information on enzyme-inhibitor interactions may be derived from n.m.r. studies.

A crude extract of HRS enzyme was used in this study. Nevertheless, it seems that this enzyme is similar in many respects to hog pancreatic PLA₂. In particular, the similar protection profiles and pH dependencies of pBPB inactivation suggest that the pBPB modification site is the same for both enzymes. Also, the lack of preferential inhibition of either enzyme by active-site directed inhibitors suggests a degree of similarity in the architecture of their catalytic centres; however, studies with more potent inhibitors may uncover differences here.