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Isolation and characterisation of sialidase from a strain of Streptococcus oralis

Joint Microbiology Research Unit, GKT Dental Institute, London SE5 9RW

Corresponding author: Dr H. L. Byers (e-mail: helen.byers{at}kcl.ac.uk).

Received 25 Feb. 1999; revised manuscript received 23 June 1999; accepted 12 Aug. 1999.

Abstract

Streptococcus oralis, the most virulent of the viridans streptococci, produces a sialidase and this exo-glycosidase has been implicated in the disease process of a number of pathogens. The sialidase of S. oralis strain AR3 was purified in order to understand the characteristics of this putative virulence determinant. The enzyme isolated as a high mol. wt aggregate (c. 325 kDa) was purified 4520-fold from late exponential phase cultures by a combination of ultrafiltration, ammonium sulphate precipitation, ion-exchange and gel filtration chromatography. The sialidase component had a mol. wt of 144 kDa as determined by SDS-PAGE analysis. The purified sialidase released N-acetylneuraminic acid from a range of sialoglycoconjugates including human ₁-acid glycoprotein, bovine submaxillary mucin, colominic acid and sialyl-2,3- and sialyl-2,6-lactose. Also, N-glycolylneuraminic acid was cleaved from bovine submaxillary mucin. The sialidase had a K_m of 11.8 µM for ₁-acid glycoprotein, was active over a broad pH range with a pH optimum of 6.0 and cleaved 2,3-, 2,6- and 2-8-sialyl glycosidic linkages with a marked preference for 2,3-linkages. The enzyme was competitively inhibited by the sialic acid derivative, 2,3-dehydro-N-acetylneuraminic acid, with a K_IC of 1.2 µm. The characteristics of the purified sialidase would support a nutritional role for this enzyme that may be significant in the proliferation of this organism in the oral cavity and at extra-oral sites in association with life-threatening infections.

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