CHARACTERIZATION AND DEVELOPMENTAL EXPRESSION OF LACTOTRIOSYLCERAMIDE, GALACTOSYLTRANSFERASE FOR THE SYNTHESIS OF NEOLACTOTETRAOSYLCERAMIDEIN THE NERVOUS-SYSTEM

Neolactoglycolipids are derived from neolactotetraosylceramide (nLcOse (4)Cer). They are found during the embryonic and neonatal developmenta l periods in the rat cerebral cortex and disappear shortly after birth . These glycolipids are, however, abundant in the adult cerebellum. La ctotriosylceramide (LcOse(3)Cer):galactosyltransferase (GT), which cat alyzes the terminal step in the biosynthesis of nLcOse(4)Cer, was char acterized in mammalian brain. The enzyme was highly specific for LcOse (3)Cer, with a terminal GlcNAc beta 1-3Gal-residue, and it did not cat alyze the transfer of galactose to other glycolipids studied with alte rnate carbohydrate residues. The microsomal membrane enzyme required M n2+ and a detergent for in vitro activity. The optimal pH was 7.4, and the K, value for LcOse(3)Cer was 34 mu M (V-max = similar to 2 nmol/m g/h). The LcOse(3)Cer:GT was shown to be different from the GM2:GT and the soluble enzyme lactose synthase A. The specific activity of LcOse (3)Ger:GT was enriched fivefold higher in the white matter than in the gray matter of young adult rat brain, whereas GM2:GT was enriched onl y about 1.5-fold higher in the white matter. The developmental express ion of LcOse(3)Cer:GT in the cerebral cortex and cerebellum was not co rrelative with the levels of nLcOse(4)Cer in these neural areas. Despi te the complete absence of nLcOse(4)Cer in the cerebral cortex of anim als older than 5 days, significant activity of the LcOse(3)Cer:GT was found even in the adult cortex. In cerebellum, the levels of nLcOse(4) Cer increased with development, but the specific activity of the enzym e was reduced by 50% soon after birth and then remained practically th e same with development. The results indicate that LcOse(3)Cer:GT is n ot a regulatory enzyme that controls the expression of nLcOse(4)Cer an d its derived neolactoglycolipids in the brain.