Inhibition of GM2 and GD3 Synthase of End-Product Gangliosides in Vitro
The effect of end-product gangliosides (GD1a,GT1b, GQ1b) on the activities of two key enzymes in ganglioside biosynthesis, namely GM2 synthase and GD3 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 GM2 synthase without disrupting the vesicular intactness; it has, however, no effect on GD3 synthase (1,2). In such a detergent-free system GM2 synthase was always found to be under substrate inhibition i.e., the activity reached a maximum at GM3 concentration of 80 µM and came sharply down with increasing concentration of GM3. At all GM3 concentrations gangliosides GD1a, GT1b, and GQ1b inhibited GM2 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.