In 1998, nSMase1 (SMPD2) was cloned and identified according to remote sequence similarity with bacterial SMase (Tomiuk et al., 1998). Human nSMase1 is a 423-amino acid protein with a predicted molecular weight of 47.6 kDa and shows significant homology with ISC1 at the amino acid sequence. Consistent with this, nSMase1 is also an integral membrane protein with two putative transmembrane domains at the C-terminus. Analysis of N-SMase activity in vitro revealed that nSMase1 was Mg2+-dependent (Tomiuk et al., 1998) and mutagenesis studies identified two histidine residues, His-136 and His-272, essential for catalysis (Rodrigues-Lima et al., 2000). Enzymatic activity required reducing agents and was reversibly inhibited by reactive oxygen species (ROS) and oxidized glutathione (GSH) (Fensome et al., 2000). The enzyme is also irreversibly inactivated by peroxynitrite, a nitric oxide-derived oxidant (Josephs et al., 2002). Northern blot analysis showed that nSMase1 mRNA is ubiquitously expressed in multiple tissues as a 1.7-kb mRNA; the ubiquitous expression pattern was further confirmed at the protein level (Tomiuk et al., 2000). The over-expressed nSMase1 colocalizes with marker proteins of the endoplasmic reticulum (ER) and the
