SUBCELLULAR-DISTRIBUTION OF N-ETHYLMALEIMIDE-SENSITIVE AND N-ETHYLMALEIMIDE-INSENSITIVE PHOSPHATIDIC-ACID PHOSPHOHYDROLASE IN RAT-BRAIN

The dephosphorylation of phosphatidic acid by phosphatidic acid phosph ohydrolase (PAP) is important in both cell-signalling and in glyceroli pid metabolism. However, these roles are apparently performed by two d ifferent enzymes, which can be distinguished by their sensitivity in v itro to N-ethylmaleimide (NEM). Both of these enzymes are present in r at brain as well as a wide range of other rat tissues. However, the qu antity and specific activity of each enzyme varies considerably betwee n different tissues, as does the ratio of the two enzymes in each tiss ue. Tissues rich in glycerolipids are abundant in NEM-sensitive PAP, w hereas there is no obvious pattern to the distribution of the NEM-inse nsitive enzyme in the different tissues tested. Studies on brain corte x, which is relatively rich in both forms of PAP, indicate that the NE M-insensitive PAP is located in the synaptosomes, and the NEM-sensitiv e enzyme present in the cytosol and microsomes. The NEM-sensitive PAP can also be translocated from the cytosol to the microsomes by oleate. When assayed against a range of phosphatidic acids, NEM-sensitive PAP showed a preference for phosphatidic acids with short acyl chains and for those containing arachidonate, whereas NEM-insensitive PAP had a preference for short and unsaturated acyl chains. The two isozymes als o had different activity profiles against these substrates suggesting that they are in fact different enzymes. The implications for these re sults on the putative roles of the two forms of PAP are discussed.