Properties of acid ceramidase from human spleen

We have characterised ceramidase activity in extracts of human spleen from control subjects and from patients with Gaucher disease. In Triton X-100 extracts of control spleens, a broad pH optimum of pH 3.5-5.0 was found; no ceramidase activity was detectable at neutral or alkaline pH. About 45-60% acid ceramidase could be extracted from spleen without detergents, but for complete extraction, Triton X-100 was required. For the radiolabelled substrate oleoylsphingosine, a K_m of 0.22 ± 0.09 mM and a V_max of 57 ± 11 nmol/h per mg protein was calculated in spleen from a control subject. Flat-bed isoelectric focussing in the presence of Triton X-100 revealed a pI of 6.0-7.0 for acid ceramidase; similar values were found for sphingomyelinase and glucerebrosidase. HPLC-gel filtration indicated that in the presence of Triton X-100, acid ceramidase has an M_r of about 100 kDa. In the absence of detergents, the enzyme forms high-molecular-weight aggregates. Similar aggregation behaviour was observed for sphingomyelinase, while the elution of β-hexosaminidase was not affected by detergents. The elution profile of glucocerebrosidase was only slightly altered by Triton X-100. There was no difference in the properties of acid ceramidase present in spleen from control subjects and from patients with type I Gaucher disease.