Purification and characterization of recombinant, human acid ceramidase: Catalytic reactions and interactions with acid sphingomyelinase

Human acid ceramidase was overexpressed in Chinese hamster ovary cells by amplification of the transfected, full-length cDNA. The majority of the overexpressed enzyme was secreted into the culture media and purified to apparent homogeneity. The purified protein contained the same 13- (α) and 40 (β)-kDa subunits as human acid ceramidase from natural sources, had an acidic pH optimum (4.5), and followed normal Michaelis-Menten kinetics using ¹⁴C- and BODIPY-labeled C₁₂-ceramide as substrates. Deglycosylation studies showed that the recombinant enzyme contained mostly "high mannose" type oligosaccharides and that two distinct β-subunits were present. Amino acid sequencing of these subunit polypeptides revealed a single N terminus, suggesting that the ∼-2-4-kDa molecular mass difference was likely due to C-terminal processing. The purified enzyme also catalyzed ceramide synthesis in vitro using ¹⁴C-labeled C12 fatty acid and sphingosine as substrates. Surprisingly, we found that media from the overexpressing hamster cells had increased acid sphingomyelinase activity and that this activity could be co-precipitated with acid ceramidase using anti-ceramidase antibodies. Overexpression of acid ceramidase in normal human skin fibroblasts also led to enhanced acid sphingomyelinase secretion, but this was not observed in Niemann-Pick disease cells. RNA studies showed that this increased activity was not due to overexpression of the endogenous acid sphingomyelinase gene. Uptake studies using mouse macrophages revealed rapid internalization of the acid ceramidase activity from the hamster cell media but not acid sphingomyelinase. These studies provide new insights into acid ceramidase and the related lipid hydrolase, acid sphingomyelinase.