Neisseria gonorrhoeae readily underwent autolysis when suspended in N-2-'hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) buffer at alkaline pH values. Autolysis was inhibited by the addition of Mg2" or other divalent cations. Autolysis was also suppressed at acid pH (pH 6.0). Suspension of cells in buffer was accompanied by the hydrolysis of peptidoglycan. The rate of peptidoglycan hydrolysis in HEPES buffer was maximal at pH 8.5 and was similar in the presence or absence of Mg2+. Therefore, divalent cation stabilization against autolysis is not mediated by inhibition of peptidoglycan hydrolysis. Peptidoglycan hydrolysis occurred in HEPES buffer (pH 6.0), but at a rate that was 50% of the maximum. Incubation of cells with chloramphenicol or rifampin before suspension in HEPES buffer (pH 8.5) partially prevented autolysis; under these conditions, peptidoglycan hydrolysis still occurred, but at a reduced rate. Old and new peptidoglycans were hydrolyzed at similar rates. Peptidoglycan hydrolysis results in solubilization of both the peptide and glycan moieties. When grown in complex media containing glucose, the peptidoglycan (PG) of virulent and avirulent gonococci constitutes 1 to 2% of the dry weight of the cell and contains muramic acid, glucosamine, alanine, glutamic acid, and diaminopimelic acid (DAP) in a ratio of 1:1:2:1:1 (9). A lipoprotein, covalently attached to the PG, has not been demonstrated in Neisseria gonorrhoeae (9, 22). The PG of this organism turns over at a high rate during exponential growth (approximately 50% per generation) (9). Hebeler and Young (10) reported that the enzyme principally responsible for this rapid turnover was N-acetylmuramyl-L-alanine amidase (EC 3.5.1.28), which cleaves the amide linkage between N-acetylmuramic acid and L-alanine. This enzyme, when extracted by salt and detergent from cell walls and assayed in vitro with [3H]DAP-labeled PG as the substrate, exhibited a broad pH optimum and was insensitive to Mg2'. No hexaminidase (endo N-acetylglucosaminidase or endo N-acetylmuramidase), which catalyzes the hydrolysis of the glycan backbone of the PG, has been reported in N. gonorrhoeae. Gonococci resuspended in buffer at an alkaline pH undergo rapid autolysis (8). This autolysis can be prevented by the addition of divalent cations or by osmotic stabilization (3, 4
Physiological conditions that would provide maximal rates of autolysis of Neisseria gonorrhoeae were examined. Autolysis was found to occur over a broad pH range with the optimum at pH 9.0 in 0.05 M tris(hydroxymethyl)aminomethane-maleate buffer. The temperature optimum was found to be 40 C. Potassium ions greatly stimulated autolysis at a concentration of 0.01 M. Exposure of growing N. gonorrhoeae cells to penicillin, vancomycin, or D-cycloserine influenced the susceptibility to the autolysin, whereas chloramphenicol afforded some protection against autolysis. The primary structure of the peptidoglycan is composed of muramic acid/glutamic acid/alanine/diaminopimelic acid/glucosamine in approximate molar ratios of 1:1:2:1:1, respectively. Exogenous radioactive diaminopimelic acid, D-glucosamine, and D-alanine were incorporated into peptidoglycan. During autolysis these radioactive fragments were released from cells.
The peptidoglycan of all four colonial types of a number of strains of Neisseria gonorrhoeae constituted 1 to 2% of the dry weight of the cell. The chemical composition of cell types examined was similar with molar ratios of 1:1:2:1:1 for muramic acid, glucosamine, alanine, glutamic acid, and diaminopimelic acid, respectively. Ninety-six percent of the mass of the peptidoglycan was composed of these compounds. A lipoprotein analogous to that observed in Escherichia coli was not detected. The chain length of the glycan varied from 80 to 110 disaccharide units. The peptide contained equimolar amounts of D- and L-alanine. The rate of turnover of peptidoglycan in strain RD5 was 50% per generation. Turnover proceeded without a lag and followed first-order kinetics.
The major autolysin(s) of Neisseria gonorrhoeae was solubilized from envelopes by extraction with 2% Triton X-100 containing 0.5 M NaCl. Neither Triton X-100 nor NaCl alone could effectively release the autolysin(s). The major autolysin is N-acetylmuramyl-L-alanine amidase (E.C. 3.5.1.28). The pH optimum for this reaction was broad, ranging from 5.5 to 8.5. Optimal hydrolysis of peptidoglycan occurred in 2% Triton X-100 in 0.1 M KCl. Attempts to purify the autolysin were unsuccessful. A rapid assay for enzyme activity was developed
Tricarboyxlic acid cycle activity was examined in Neisseria gonorrhoeae CS-7. The catabolism of glucose in N. gonorrheae by a combination of the Entner-Doudoroff and pentose phosphate pathways resulted in the accumulation of acetate, which was not further catabolized until the glucose was depleted or growth became limiting. Radiorespirometric studies revealed that the label in the 1 position of acetate was converted to CO2 at twice the rate of the label in the 2 position, indicating the presence of a tricarboxylic acid cycle. Growth on glucose markedly reduced the levels of all tricarboxylic acid cycle enzymes except citrate synthase (EC 4.1.3.7). Extracts of glucose-grown cells contained detectable levels of all tricarboxylic acid cycle enzymes except aconitase (EC 4.2.1.3), isocitrate dehydrogenase (EC 1.1.1.42), and a pyridine nucleotide-dependent malate dehydrogenase (EC 1.1.1.37). Extracts of cells capable of oxidizing acetate lacked only the pyridine nucleotide-dependent malate dehydrogenase. In lieu of this enzyem, a particulate pyridine nucleotide-independent malate oxidase (EC 1.1.3.3) was present. This enzyme required flavin adenine dinucleotide for activity and appeared to be associated with the electron transport chain. Radiorespirometric studies utilizing labeled glutamate demonstrated that a portion of the tricarboxylic acid cycle functioned during glucose catabolism. In spite of the presence of all tricarboxylic acid cycle enzymes, N. gonorrhoeae CS-7 was unable to grow in medium supplemented with cycle intermediates.
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