Sphingomyelin Phosphodiesterase 3 (SMPD3)

The predicted 655-amino acid SMPD3 protein has a molecular mass of 70 kD and is a member of a large superfamily of magnesium-dependent phosphohydrolases. SMPD3 contains an N-terminal membrane-anchoring domain and a C-terminal catalytic domain. The catalytic domains of SMPD3 and SMPD2 share 23% sequence identity. Northern blot analysis detected a 6-kb SMPD3 transcript in brain and liver, as well as an additional 3.5-kb transcript at low levels in thymus. Expression was most prominent in large cells, including Purkinje cells, pyramidal cells, neurons of the dentate gyrus granular layer, and neurons in the pontine nuclei. SMPD3 has a neutral pH optimum and specifically hydrolyzes sphingomyelin with no detectable phospholipase C like activity.