The enzymic synthesis of GM1b: rat-brain CMP-N-acetylneuraminic acid: asialo-GM1 sialyltransferase.

An enzyme which catalyzes the transfer of N-acetylneuraminic acid (NeuNAc) to a tetrahexosylceramide (asialo-GM1) in young rat brain is described. The enzymic product is a new monosialoganglioside containing a neuraminidase-labile neuraminic acid, GM1b. The activity of this sialyltransferase is higher in fetal and young rat brains. The enzyme exhibits a pH optimum of 6.5 in cacodylate buffer. The incorporation of radioactivity into GM1b is stimulated in the presence of asialo-GM1 and CMP-NeuNAc and is dependent on the quantity added. The detergent mixture, Tween 80 and CF54, is required for optimal activity. Recent demonstration of the natural occurrence of GM1b in the free cell types of rat ascites hepatopa cells suggests a functional importance of this CMP-Neu-NAc:asialo-GM1 sialyltransferase in the in vivo formation of this novel monosialoganglioside.