40] Serine-ethanolamine base-exchange enzyme from rat brain

Abstract

Publisher Summary This chapter discusses the serine–ethanolamine base-exchange enzyme from rat brain. The base-exchange enzyme activities are widely distributed in mammalian cell membranes. These base-exchange activities do not result in the net synthesis of a phospholipid; they merely catalyze an exchange of the polar amino alcohol or L-serine of preexisting phospholipids. The de novo pathway of phospholipid biosynthesis involving phosphorylated and cytidine nucleotide derivatives are the principal route for formation of the phospholipids possessing choline or ethanolamine or mono-methylethanolamine or dimethylethanolamine as the characteristic polar head groups. However, the serine base-exchange enzyme is the sole mechanism available for phosphatidylserine formation by mammalian tissues. The assays for the base-exchange activities depend on solubility differences of the precursor amino alcohol employed as substrate and the product phospholipid corresponding to the given amino alcohol. The most frequently used approach based on this physical difference is a partitioning of an aqueous and an organic solution. Utilizing a radioactive water-soluble precursor, which is quantitatively retained in the aqueous phase, the amount of radioactivity appearing in the organic phase is an accurate reflection of the base-exchange enzyme activity of biological sample. Confirmation of the reliability of this approach is obtained from thin-layer chromatographic techniques demonstrating cochromatography of the expected product with authentic standards. Reliable results can also be obtained by estimating the quantity of radioactivity present in a trichloroacetic acid (TCA) precipitate of an incubation mixture.