Secretory Sphingomyelinase sSMase

The first nSMases of mammalian origin were characterized by Gatt [42] and by Rao and Spence [43]. Liu et al. [44] purified by 3030-fold a Mg -dependent, nSMase activity (ca. 60 kDa) from rat brain membranes. Enzyme activity was highly dependent on phosphatidylserine, and it was 95% inhibited by 3 mM glutathione [45]. Glutathione inhibition suggested fhat nSMase was activated by stress [46]. More recenfly, Bernardo et al. [47] have purified by 23 330-fold a Mg -dependent nSMase from bovine brain fhat was inhibited by glutathione. The en- 

Ghosh et al. [48] have isolated two isoforms of nSMase from rabbit skeletal muscle (92 and 53 kDa). Peptide mapping revealed important structural similarities, and the catalytic activities were also similar, except that the 53 kDa protein was Mg -independent. These nSMases are located in the transverse tubules of the muscle cells, which may be related to the observation that sphingosine modulates calcium release from sarcoplasmic reticulum membranes [49]. Two Mn -and Mg -dependent nSMases located in the microsomal membranes of seminiferous tubes of immature Wistar rats have been characterized [50] whose properties do not appear to differ significantly from other mammalian nSMases. Finally, we mention two other nSMases purified from eukaryotic natural sources, namely the Mg -dcpcridcril nSMase isolated from Saccharomyces cerevisiae [51], and that obtained from membrane fractions of intraeryfhrocytic Plasmodium falciparum, the malaria parasite [52]. The latter enzyme was activated by phosphatidylserine and other anionic phosphohpids, and was sensitive to scyphostatin, an inhibitor of mammalian nSMase (see below). 

Subsequently, Tomiuk et al. [56] also concluded that the enzyme they had previously cloned (nSMase 1 in their nomenclature) was predominantly localized in the microsomal fraction (endoplasmic reticulum and Golgi). anti-nSMase 1 antibodies did not affect the nSMase activity in membrane fractions from murine brain [56], indicating that nSMase 1 was one of at least two mammalian nSMases. Indeed, shortly afterwards, the same group published the cloning and characterization of a 71 kDa mammalian brain-specific, Mg -dependent nSMase [57], 

The identification of a nSMase activity in LDL is significant [69, 70[. These authors have demonstrated that apolipoprotein B-lOO, the protein moiety of LDL, 

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