There has been growing interest in incorporating single-wall carbon nanotubes (SWNTs) as toughening agents in brittle ceramics. Here we have prepared dense Al(2)O(3)/SWNT composites using the spark-plasma sintering (SPS) method. Vickers (sharp) and Hertzian (blunt) indentation tests reveal that these composites are highly contact-damage resistant, as shown by the lack of crack formation. However, direct toughness measurements, using the single-edge V-notch beam method, show that these composites are as brittle as dense Al(2)O(3) (having a toughness of 3.22 MPa m(0.5)). This type of unusual mechanical behaviour was also observed in SPS-processed, dense Al(2)O(3)/graphite composites. We argue that the highly shear-deformable SWNTs or graphite heterogeneities in the composites help redistribute the stress field under indentation, imparting the composites with contact-damage resistance. These composites may find use in engineering and biomedical applications where contact loading is important.

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The distribution of bacterial L-methionine gamma-lyase (L-methionine methanethiollyase (deaminating) (EC 4.4.1.11) was investigated, and Pseudomonas ovalis (IFO 3738) was found to have the highest activity of enzyme, which was inducibly formed by addition of L-methionine to the medium. L-Methionine gamma-lyase, purified to homogeneity from Ps. ovalis, has a molecular weight of about 173 000 and consists of nonidentical subunits (mol wt: 40 000 and 48 000). The enzyme exhibits absorption maxima at 278 and 420 nm, and a shoulder around 330 nm, which are independent of the pH (6.0 to 10.0), and contains 4 mol of pyridoxal 5'-phosphate per mol of the enzyme. The formyl group of pyridoxal 5'-phosphate is bound in an aldimine linkage to the epsilon-amino group of lysine residues of the protein. The holoenzyme is resolved to the apoenzyme by incubation with hydroxylamine, and reconstituted by addition of pyridoxal 5'-phosphate. The enzyme activity is significantly affected by both carbonyl and sulfhydryl reagents. L-Methionine gamma-lyase catalyzes alpha,gamma- and alpha,beta-elimination reactions of, in addition to L-methionine, several derivatives of L-methionine and L-cysteine, e.g., L-ethionine, DL-methionine sulfone, L-homocysteine, and S-methyl-L-cysteine. The enzyme catalyzes also gamma-replacement reactions of the thiomethyl group of methionine with various alkanethiols (C2-C7), arylthio alcohols (benzenethiol and beta-naphthalenethiol) and the derivatives of ethanethiol (2-mercaptoethanol and cysteamine) to yield the corresponding S-substituted homocysteine. The thiomethyl group of S-methyl-L-cysteine also is replaced by ethanethiol to form S-ethyl-L-cysteine.

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