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: G_M3, G_D3, G_D1a, G_T1, G_Q1 are affected by the enzyme and disappear almost completely after 4 hr of incubation. In contrast, a ganglioside migrating like G_M1 accumulates and does not seem to be hydrolysed. G_D1b undergoes a transient increase within 1 hr which is consistent with the corresponding decrease of G_T1. By comparing the absolute quantities of gangliosides that disappeared after 30 min of incubation, it is shown that G_D1a 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.