developmental studies of the catabolism of sialocompounds by endogenous neuraminidase in the chick retina

The effect of a "particle bound" neuraminidase acting on endogenous sialocompounds (glycoproteins and gangliosides) in chick retina during development has been investigated. The total "particle bound" N-acetyl neuraminic acid (NeuNAc) per gram wet weight, increased fourfold between 8-day-old embryos and hatching. The ratio of ganglioside to glycoprotein-bound NeuNAc changed during retina ontogenesis: it approached 5 for the 8-day-old embryo, decreased to approximately 2 in the 15-day embryo and was approximately one from hatching to adulthood. The lowest neuraminidase activity was observed in the 8-day-old embryo and the highest was obtained at hatching. For each age of retina and for each incubation time 90% of the NeuNAc released by neuraminidase action was of ganglioside origin; the ganglioside-bound NeuNAc therefore seems to be a more accessible substrate for the chick retinal endogenous neuraminidase than that bound to glyco-proteins. The release of NeuNAc from gangliosides leads to drastic changes in the ganglioside patterns: GM3, GD3, GD1a, GT1, GQ1 are affected by the enzyme and disappear almost completely after 4 hr of incubation. In contrast, a ganglioside migrating like GM1 accumulates and does not seem to be hydrolysed. GD1b undergoes a transient increase within 1 hr which is consistent with the corresponding decrease of GT1. By comparing the absolute quantities of gangliosides that disappeared after 30 min of incubation, it is shown that GD1a is the main glycolipid attacked by the enzyme. The greater hydrolysis of gangliosides compared to the particulate glycoproteins may indicate that gangliosides may play a greater role in mechanisms which require sialic acid movements than that played by the glycoproteins. The possible role of the "particle bound" neuraminidase in the retina is discussed.