[Secretary sialidase activity and GM3 ganglioside]

The sialidase activities with GM3 ganglioside and sialyllactitol were demonstrated in the conditioned medium of human fibroblasts. pH versus activity profiles of conditioned medium with GM3 as substrate suggested the presence of two sialidases with optimal activities at pH 4.5 and pH 6.5. The GM3 sialidase activity at pH 6.5 was suppressed in the medium of contact-inhibited cells. This sialidase may function in the metabolism of cell surface GM3 since there was a selective loss of labeled sialic acid from GM3 at different times of incubation after pulse-labeling with a radioactive sialic acid precursor ([3H]N-acetyl-mannosamine) and a radioactive ceramide precursor ([14C]serine). In addition, a sialidase inhibitor, 2-deoxy-2, 3-dehydro-N-acetyl-neuraminic acid (NeuAc-2-en) resulted in a reversible growth inhibitory effect and the suppression of the sialidase activity in the medium. We have speculated that GM3 hydrolysis on the cell surface by the sialidase may be coordinated with the cell cycle and may be at its maximum during early in the G1 phase.