A SAPOSIN-LIKE DOMAIN INFLUENCES THE INTRACELLULAR-LOCALIZATION, STABILITY, AND CATALYTIC ACTIVITY OF HUMAN ACYLOXYACYL HYDROLASE

Acyloxyacyl hydrolase, a leukocyte enzyme that acts on bacterial lipop olysaccharides (LPSs) and many glycerolipids, is synthesized as a prec ursor polypeptide that undergoes internal disulfide linkage before bei ng proteolytically processed into two subunits. The larger subunit con tains an amino acid sequence (Gly-X-Ser-X-Gly) that is found at the ac tive sites of many lipases, while the smaller subunit has amino acid s equence similarity to saposins (sphingolipid activator proteins), cofa ctors for sphingolipid glycohydrolases. We show here that both acyloxy acyl hydrolase subunits are required for catalytic activity toward LPS and glycerophosphatidylcholine. In addition, mutations that truncate or delete the small subunit have profound effects on the intracellular localization, proteolytic processing, and stability of the enzyme in baby hamster kidney cells. Remarkably, proteolytic cleavage of the pre cursor protein increases the activity of the enzyme toward LPS by 10-2 0-fold without altering its activity toward glycerophosphatidylcholine . Proper orientation of the two subunits thus seems very important for the substrate specificity of this unusual enzyme.