A comparison of the reversibility of phosphoethanolamine transferase and phosphocholine transferase in rat brain microsomes.

The reversibility of phosphoethanolamine transferase (EC 2.7.8.1) in rat brain is demonstrated in this paper. Microsomal ethanolamine glycerophospholipids were prelabeled with an intracerebral injection of [3H]ethanolamine 4 h before killing young rats. Labeled CDPethanolamine was produced by incubation of the microsomes with CMP, although to a lesser extent than for the previously observed release of CDPcholine. Ethanolamine and choline glycerophospholipids were labeled with [2-3H]glycerol by incubation with primary cultures of rat brain. Microsomes from rat brains, with diisopropyl phosphofluoridate for inhibition of lipases, were incubated with the labeled glycerophospholipids separately, and labeled diacylglycerols were produced. The kinetic parameters of phosphoethanolamine transferase and phosphocholine transferase (EC 2.7.8.2) were compared by incubating rat brain microsomes with [3H]CMP. Inclusion of AMP in the reaction mixture was necessary in order to inhibit the hydrolysis of CMP by an enzyme with the properties of 5'-nucleotidase (EC 3.1.3.5). For phosphoethanolamine transferase and phosphocholine transferase respectively, the Km values for CMP were 40 and 125 microM and the V values were 2.3 and 21.6 nmol/h per mg protein. The reversibility of both enzymes permits the interconversion of the diacylglycerol moieties of choline and ethanolamine glycerophospholipids. During brain ischemia, a principal pathway for degradation of ethanolamine glycerophospholipids may be by reversal of phosphoethanolamine transferase followed by hydrolysis of diacylglycerols by the lipase.