A newly isolated periphytic marine bacterium has been shown to synthesize a true tyrosinase. The enzyme exhibited both cresolase and catecholase functions and catalyzed the biosynthesis of melanin from l‐tyrosine. Enzyme activity was enhanced in the presence of oxidants and was inhibited by copper chelating agents such as diethyldithiocarbamic acid and cyanide. The apparent molecular weigth of the 2–40 tyrosinase (67 000) makes this enzyme the largest known procaryote tyrosinase.

Nine strains of Streptococcus oralis, isolated from blood cultures of patients with infective endocarditis or from the oral cavity as part of the normal flora, were examined for their ability to elaborate sialidase (neuraminidase) and N-acetylglucosaminidase, enzymes which are involved in the degradation of glycoproteins. Both glycosidases were induced when bacteria were grown in a minimal medium supplemented with porcine gastric mucin, a model glycoprotein, and repressed when growth occurred in the presence of glucose. Cell-free extracts of mucin-grown cultures expressed elevated levels of N-acetylneuraminate pyruvate-lyase (the first intracellular enzyme in the pathway of N-acetylneuraminate catabolism), N-acetylglucosamine (GlcNAc)-6-phosphate deacetylase and glucosamine-6-phosphate deaminase (enzymes involved in the intracellular catabolism of GlcNAc 6-phosphate); activity of each of these intracellular enzymes was markedly repressed when bacteria were grown in the presence of glucose. Three strains of 5. oralis were also grown in media supplemented with a,-acid glycoprotein, a major component of human plasma. Cells from these cultures expressed high levels of sialidase, N-acetylglucosaminidase, and the intracellular enzymes involved in the catabolism of N-acetyl-sugars released by the action of these glycosidases. High-resolution 'H-NMR spectroscopy of spent culture supernatants revealed that sialic acid and GlcNAc residues of the molecularly mobile oligosaccharide side-chains of &,-acid glycoprotein had been hydrolysed and the released sugars internalized b y the bacteria. These data indicate that S. oralis has the ability to hydrolyse constituents of oligosaccharide side-chains of host-derived glycoproteins and to utilize simultaneously these released carbohydrates. The biochemical characteristics induced by the growth of 5. oralis on glycoproteins may play a role in the survival and persistence of these bacteria a t the infection site in vivo.

Nine Streptococcus oralis strains isolated from cases of endocarditis (n=5) and from the normal oral flora (n=4) were examined for their ability to produce a number of protease activities measured using the following 7-amido-4-methylcoumarin (-AMC) linked fluorogenic substrates: BOC-leu-ser-thr-arg-AMC, gly-pro-AMC, CBZlYs-AMC, arg-AMC and BOC-Val-pro-arg-AMC, a synthetic substrate for thrombin. The influence of glucose and porcine gastric mucin on their production was determined. The distribution of the protease activities between cell-associated and supernatant was not significantly associated with the origin of the strains. However, BOC-leuser-thr-arg-AMC and BOC-Val-pro-arg-AMC hydrolysing activity was greater in the supernatant while the gly-pro-AMC, CBZ-lys-AMC and arg-AMC hydrolysing activity was more cell associated, irrespective of whether the cells were grown in minimal media supplemented with either glucose or PGM. Inclusion of increasing concentrations of glucose in media containing a constant PGM concentration (0.25 per cent w/v) resulted in significant reductions in the supernatant protease activity hydrolysing BOC-leu-ser-thr-arg-AMC and BOC-val-proarg-AMC while the cell-associated activity hydrolysing CBZ-lys-AMC and arg-AMC increased and the hydrolysis of gly-pro-AMC was essentially unaltered. The response of S. oralis proteolytic activities to changes in media composition, including the inclusion of a model glycoprotein, PGM, was not predictable but indicated that strains from endocarditis and from the normal oral flora were indistinguishable. The production of proteases in vivo may depend upon the level of fermentable carbohydrates in the circulation but at low concentrations elevated levels of protease activity, including a thrombin-like activity, may be found within fibrin-platelet thrombi associated with endocarditis.

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