Title : A saposin-like
domain influences the intracellular localization, stability, and catalytic activity of human
acyloxyacyl hydrolase
Abstract :
- Acyloxyacyl hydrolase , a leukocyte enzyme that acts on bacterial lipopolysaccharides (LPSs) and many glycerolipids, is synthesized as a precursor polypeptide that undergoes internal disulfide linkage before being proteolytically processed into two subunits
- The larger subunit contains an amino acid sequence ( Gly-X-Ser-X-Gly ) that is found at the active sites of many lipases, while the smaller subunit has amino acid sequence similarity to saposins (sphingolipid activator proteins ), cofactors for sphingolipid glycohydrolases
- We show here that both acyloxyacyl 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 precursor protein increases the activity of the enzyme toward LPS by 10-20-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