Characterization and Developmental Expression of Lactotriosylceramider: Galactosyltransferase for the Synthesis of Neolactotetraosylceramide in the Nervous System

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Abstract
Neolactoglycolipids are derived from neolactotetraosylceramide (nLcOse4Cer). They are found during the embryonic and neonatal developmental periods in the rat cerebral cortex and disappear shortly after birth. These glycolipids are, however, abundant in the adult cerebellum. Lactotriosylceramide (LcOse3Cer):galactosyltransferase (GT), which catalyzes the terminal step in the biosynthesis of nLcOse4Cer, was characterized in mammalian brain. The enzyme was highly specific for LcOse3Cer, with a terminal GlcNAc beta 1-3Gal-residue, and it did not catalyze the transfer of galactose to other glycolipids studied with alternate carbohydrate residues. The microsomal membrane enzyme required Mn2+ and a detergent for in vitro activity. The optimal pH was 7.4, and the Km value for LcOse3Cer was 34 microM (Vmax = approximately 2 nmol/mg/h). The LcOse3Cer:GT was shown to be different from the GM2:GT and the soluble enzyme lactose synthase A. The specific activity of LcOse3Cer:GT was enriched fivefold higher in the white matter than in the gray matter of young adult rat brain, whereas GM2:GT was enriched only about 1.5-fold higher in the white matter. The developmental expression of LcOse3Cer:GT in the cerebral cortex and cerebellum was not correlative with the levels of nLcOse4Cer in these neural areas. Despite the complete absence of nLcOse4Cer in the cerebral cortex of animals older than 5 days, significant activity of the LcOse3Cer:GT was found even in the adult cortex. In cerebellum, the levels of nLcOse4Cer increased with development, but the specific activity of the enzyme was reduced by 50% soon after birth and then remained practically the same with development.(ABSTRACT TRUNCATED AT 250 WORDS)