Dihydrodipicolinate synthase (EC 4.2.1.52), the first enzyme unique to lysine biosynthesis in bacteria and higher plants, has been purified to homogeneity from etiolated pea (Pisum sativum) seedlings using a combination of conventional and affinity chromatographic steps. This is the first report on a homogeneous preparation of native dihydrodipicolinate synthase from a plant source. The pea dihydrodipicolinate synthase has an apparent molecular weight of 127,000 and is composed of three identical subunits of 43,000 as determined by gel filtration and crosslinking experiments. The tnmenc quaternary structure resembles the trimenc structure of other aldolases, such as 2-keto-3-deoxy-6-phosphogluconic acid aldolase, which catalyze similar aldol condensations. The amino acid compositions of dihydrodipicolinate synthase from pea and Escherichia coli are similar, the most significant difference concems the methionine content: dihydrodipicolinate synthase from pea contains 22 moles of methionine residue per mole of native protein, contrary to the E. coli enzyme, which does not contain this amino acid at all. Dihydrodipicolinate synthase from pea is highly specific for the substrates pyruvate and L-aspartate-fl-semialdehyde; it follows Michaelis-Menten kinetics for both substrates. The pyruvate and L-aspartate-ft-semialdehyde have Michaelis constant values of 1.70 and 0.40 millimolar, respectively. L-Lysine, S-(2-aminoethyl)-L-cysteine, and La-(2-aminoethoxyvinyl)glycine are strong allostenc inhibitors of the enzyme with 50% inhibitory values of 20, 160, and 155 millimolar, respectively. The inhibition by L-lysine and L-a-(2-aminoethoxyvinyl)glycine is noncompetitive towards L-aspartateBl-semialdehyde, whereas S-(2-aminoethyl)-L-cysteine inhibits dihydrodipicolinate synthase competitively with respect to L-aspartate-jt-semialdehyde. Furthermore, the addition of (2R,3S,6S)-2,6-diamino-3-hydroxy-heptandioic acid (1.2 millimolar) and (2S,6R/S)-2,6-diamino-6-phosphono-hexanic acid (1.2 millimolar) activates dihydrodipicolinate synthase from pea by a factor of 1.4 and 1.2, respectively. This is the first reported activation process found for dihydrodipicolinate synthase.

No abstract

The influence of the safener, CGA 185072 (5-chloro-8-quinolinoxy-acetic acid-1-methylhexyl-ester), on the metabolism of the aryloxyphenoxypropanoate herbicide, CGA 184927 (2-propynyl-R-2-[4-(5-chloro-3-fluoro-2-pyridinyloxy)-phenoxy]-propionate), was studied in excised leaves of wheat, barley, and maize. In wheat and barley, CGA 184927 readily under­went ester hydrolysis followed by hydroxylation at the pyridinyl moiety as well as ether cleav­age between the pyridinyl and the phenyl ring. Ether cleavage constituted the minor pathway in both species. All metabolites were subject to glycosyl conjugation. Tetcyclacis strongly in­hibited pyridinyl-ring hydroxylation in wheat. Metabolism by hydroxylation and ether cleav­age was more rapid in wheat than in barley, and was found to be accelerated in the presence of the safener CGA 185072 in both wheat and, to a lesser degree, in barley. Moreover, the safener increased the capacity for O-glycoside formation in wheat as suggested from studies using the ,4C-labelled pyridinyl-ring hydroxylated metabolite as a precursor. In maize, which is highly susceptible to CGA 184927, rapid ester hydrolysis of CGA 184927 and partial conversion of the corresponding carboxylic acid to glycosyl ester conjugate(s) occurred. However, no further transformation of the herbicide was found in maize, both in the absence or presence of CGA 185072. It is concluded that the ability of CGA 185072 to protect wheat from injury by the herbicide, CGA 184927, and to confer partial protection to barley, is related to the ability of the safener to stimulate herbicide metabolism in these crop species.

Flower formation in stem explants was chosen as an experimental system for the study of the function of hydroxycinnamoyl amides (HCAs) in plants. The explants, derived from flowering and non‐flowering Nicotiana tabacum L. var. Xanthi nc., differentiated to two types of callus and afterwards to flower buds. A novel reversed‐phase high performance liquid chromatography method, following sample clean‐up on CM‐Fractogel columns, enabled us to examine HCA concentrations in small tissue samples. Two different groups of HCA could be distinguished during in vitro flower formation: Firstly, feruloyl‐ and diferuloyputrescine, the major detectable HCAs that accumulated during callus proliferation; secondly, caffeoylputrescine, which accumulated during the later stage of flower differentiation and reached higher concentrations than feruloylputrescine.

The preparation of the four stereoisomers of propiconazole (TILT®) is described. Their inhibition of the 14α-C-demethylation of the sterol nucleus is examined and compared with the inhibition by the four stereoisomers of etaconazole (SONAX®). The quantitative structure-activity relationships (QSAR) of substituted 1,3-dioxolane-2-yl-methyltriazoles and 1,3-dioxane-2-ylmethyltriazoles on in vivo fungicidal activity are investigated.