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Biomedical subjects

H R Perkins

Publications and source records attributed to H R Perkins.

At least 37 records · Page 2Linked to original sources
PubMed1979

Effects of lysozyme on Bacillus cereus 569: rupture of chains of bacteria and enhancement of sensitivity to autolysins.

Bacillus cereus 569 is known to be resistant to lysis by lysozyme because of the presence of deacetylated glucosamine residues in its peptidoglycan, and cultures continued to grow even in the presence of lysozyme at 200 microgram ml-1. However, lysozyme caused rupture of the chains of bacteria and promoted the rate of autolysis in a non-growing cell suspension, causing a doubling of the rate of release of radioactively labelled wall material. Heat-inactivated cells did not autolyse and were not lysed by lysozyme unless they were supplemented by unheated cells or cell-free autolysate. Enhancement of autolysin activity could also be effected by pre-treatment of heated cells with lysozyme. The action of lysozyme on isolated cell walls released some free reducing groups, indicating limited breakage of the polysaccharide chains of peptidoglycan, and it was concluded that lysozyme modified the peptidoglycan and made it more susceptible to autolysin(s). Lysozyme also enhanced the rate of septum separation and the probable significance of the results in relation to the control of cell separation is discussed.

Bacillus cereus↗
PubMed1979

In vitro synthesis of peptidoglycan by beta-lactam-sensitive and -resistant strains of Neisseria gonorrhoeae: effects of beta-lactam and other antibiotics.

The synthesis in vitro of peptidoglycan by Neisseria gonorrhoeae was studied in organisms made permeable to nucleotide precursors by treatment with ether. Optimum synthesis occurred at 30 degrees C in tris(hydroxymethyl)aminomethane-maleate buffer (0.05 M; pH 6) in the presence of 20 mM Mg(2+). The incorporation from uridine 5'-diphosphate-N-acetyl-[(14)C]glucosamine into peptidoglycan, measured after precipitation of the cells with trichloroacetic acid, was sensitive to the beta-lactam antibiotics, bacitracin, diumycin, and tunicamycin and relatively resistant to spectinomycin and tetracycline. Differences in sensitivity between preparations from a beta-lactamase producer and a laboratory segregant derived from it were not great. Synthesized peptidoglycan was also fractionated into sodium dodecyl sulfate-soluble and -insoluble portions. beta-Lactam antibiotics at concentrations equivalent to the minimal inhibitory concentrations for growth of the organisms did not inhibit peptidoglycan synthesis, but rather caused a small enhancement. At higher concentrations, above about 0.5 mug/ml, incorporation into sodium dodecyl sulfate-insoluble material was progressively inhibited, whereas the amount of sodium dodecyl sulfate-soluble product increased greatly, more than compensating for the loss of the precipitable fraction. Similar observations were made with three strains, and also with the beta-lactam clavulanic acid, normally considered as a beta-lactamase inhibitor rather than as itself an effective antibiotic.

Anti-Bacterial Agents↗
PubMed1977

The peptidoglycan crosslinking enzyme system in Streptomyces strains R61, K15 and rimosus.

The DD-carboxypeptidase-transpeptidase enzyme system in Streptomyces strain K15 consists of: (1) a membrane-bound transpeptidase capable of performing low DD-carboxypeptidase activity; and (2) a set of DD-carboxypeptidases: (a) membrane-bound, (b) lysozyme-releasable and (c) exocellular, having low transpeptidase activities in aqueous media and at low acceptor concentrations. The DD-carboxypeptidases are related to each other and may belong to the same pathway leading to enzyme excretion. A similar enzyme system occurs in Streptomyces strain R61 except that the membrane-bound DD-carboxypeptidase activity is low when compared with the membrane-bound transpeptidase activity. In Streptomyces rimosus the enzyme system consists almost exclusively of the membrane-bound transpeptidase and the levels of membrane-bound, lysozyme-releasable and exocellular DD-carboxypeptidases are very low.

Carboxypeptidases↗
PubMed1977

The exchange reaction of peptides R-D-alanyl-D-alanine with D-[14C]alanine to R-D-alanyl-D-[14C]alanine and D-alanine, catalysed by the membranes of Streptococcus faecalis ATCC 9790.

Under alkaline conditions, the membrane-bound DD-carboxypeptidase of Streptococcus faecalis ATCC 9790 catalyses exchange reactions in which the X-L-R3-D-Ala moiety of peptides of the type X-L-R3-D-Ala-D-Ala is transferred to simple amino compounds such as D-alanine, glycine and glycyl-glycine. The enzyme system is unable, however, to catalyse complex reactions that would simulate the natural transpeptidation reaction.

Alanine↗
PubMed1975

Interaction between the exocellular DD-carboxypeptidase-transpeptidase from Streptomyces R61, substrate and beta-lactam antibiotics. A choice of models.

The interaction between the exocellular DD-carboxypeptidase-transpeptidase of Streptomyces R61 and beta-lactam antibiotics is a multistep process during which a rather stable enzyme - antibiotic complex is formed. This mechanism of interaction is compatible with Lineweaver-Burk plots that are typical of a competitive inhibition of the hydrolysis of the peptide donor by the antibiotic. In fact, however, the same Lineweaver-Burk plots can be obtained on the basis of a non-competitive type of inhibition. At present, a choice between the two models cannot be made.

Acyltransferases↗
PubMed1975

The catalytic activity and penicillin sensitivity in the liquid and frozen states of membrane-bound and detergent-solubilised transpeptidase of Streptomyces R61.

The Km, app. values of the membrane-bound transpeptidase of Streptomyces R61 for the donor Ac2-L-Lys-D-Ala-D-Ala and the acceptor Gly-Gly are not affected by temperature variations when the reaction mixtures are incubated in liquid suspensions. At -5 degrees C, the incubation can be carried out either in the liquid or in the frozen state. The enzyme is active in the latter state. In the frozen state, the Km, app. value for the acceptor remains unchanged but there is a 3-fold increase in the maximum velocity, a 10-fold decrease of the Km, app. value for the donor and a 10-fold increase of the benzylpenicillin concentration required to inhibit the enzyme activity by 50% (ID50 value). Temperatures of -35 degrees C or below are required to completely inhibit the membrane-bound enzyme in the frozen state. Cetyltrimethylammonium bromide extracts the transpeptidase both from the isolated membranes and, with a much higher yield, from the intact mycelium. The extracted enzyme is not active in the frozen state, requires detergent for activity, has decreased Km, app. values for both donor and acceptor, exhibits the same sensitivity to benzylpenicillin and cephalosporin C as the membrane-bound transpeptidase (in liquid suspensions) and, like this latter enzyme, has no DD-carboxypeptidase activity. The detergent-extracted transpeptidase penetrates gels of Sephadex-100 and is not sedimented at 200 000 X g.

Acyltransferases↗
PubMed1975

The specificity of enzymes adding amino acids in the synthesis of the peptidoglycan precursors of Corynebacterium poinsettiae and Corynebacterium insidiosum.

A soluble extract from Corynebacterium poinsettiae able to synthesize the nucleotide precursor of ite peptidoglycan was prepared. This extract contained all the enzymes necessary for the synthesis of the peptide side-chain. The spedificity of these enzymes was determined and compared with the specificity of similar enzymes extracted from the closely related Corynebacterium insidiosum. In both organsims, addition of the third amino acid of the peptide side-chain was specific for the amino acid and nucleotide dipeptide involved in peptidoglycan synthesis in the parent organism. L-Diaminobutyric acid, which is found as the acetyl derivative in the precursor nucleotide and in the completed peptidoglycan of C. insidiosum, was added as the free amino acid and not as the acetylated compound.

Alanine↗