Synthesis of galactosyl ceramide and glucosyl ceramide by rat brain: assay procedures and changes with age.

Galactosyl ceramide is made in mouse brain from ceramide and uridine diphospho galactose (UDPGal)16,1L The enzyme system, evaluated in vitro, shows a marked preference for 2-hydroxy fatty acids (HFA) ceramide when compared with nonhydroxy fatty acids (NFA) ceramide, although brain contains HFA and NFA cerebroside in similar amounts. We do not yet know whether this difference is due to enzyme specificity or to the existence of two different enzymes. This reaction and its unexpected specificity have been observed independently in chick brain 2 and confirmed in mouse kidney 6 and brain 1°. Glucosyl ceramide is made by a similar sugar transferase in chick brain 1 but occurs in brain only in trace amounts. It appears to be converted to gangliosides 1,22, which contain only NFA. The fatty acid composition of gangliosides differs greatly from that of galactosyl ceramide; the former contains primarily stearic acid, while the latter also contains much of the very long fatty acids. Moreover the long chain base composition of gangliosides and galactosyl ceramide also differs appreciably. Thus it appears likely that the ceramide used as substrate by the galactosyltransferase is much different than the ceramide used by the glucosyltransferase. The existence of this enzyme in mouse brain has been demonstratedlL Neither transferase has received much study and assay conditions have not yet been worked out. In this paper we report the development of assay procedures for the two enzymes, a comparison of their properties, and an evaluation of their activities in the brains of developing rats. Since the completion of this study a report has appeared 21 covering some of the same ground; some differences in the findings are discussed.