Inhibition of GM2 and GD3 Synthase of End-Product Gangliosides in Vitro

Abstract

The effect of end-product gangliosides (G_D1a,G_T1b, G_Q1b) on the activities of two key enzymes in ganglioside biosynthesis, namely G_M2 synthase and G_D3 synthase in rat liver Golgi apparatus, has been investigated in both detergent-free and detergent-containing assays. In detergent-free intact Golgi vesicles phosphatidylglycerol was used as a stimulant. This phospholipid was earlier shown to stimulate the activity of G_M2 synthase without disrupting the vesicular intactness; it has, however, no effect on G_D3 synthase (1,2). In such a detergent-free system G_M2 synthase was always found to be under substrate inhibition i.e., the activity reached a maximum at G_M3 concentration of 80 µM and came sharply down with increasing concentration of G_M3. At all G_M3 concentrations gangliosides G_D1a, G_T1b, and G_Q1b inhibited G_M2 synthase strongly. This inhibition appeared to increase with increase in the number of sialyl groups (increase of negative charge) of the inhibiting gangliosides and therefore seemed to be an unspecific physical effect.