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M H Zenk

Publications and source records attributed to M H Zenk.

At least 73 records · Page 4Linked to original sources
PubMed1987

Phytochelatins, a class of heavy-metal-binding peptides from plants, are functionally analogous to metallothioneins.

Phytochelatins are a class of heavy-metal-binding peptides previously isolated from cell suspension cultures of several dicotyledonous and monocotyledonous plants. These peptides consist of repetitive gamma-glutamylcysteine units with a carboxyl-terminal glycine and range from 5 to 17 amino acids in length. In the present paper we show that all plants tested synthesized phytochelatins upon exposure to heavy metal ions. No evidence for the occurrence of metallothionein-like proteins was found. All data so far obtained indicate that phytochelatins are involved in detoxification and homeostasis of heavy metals in plants and thus serve functions analogous to those of metallothioneins in animals and some fungi. Phytochelatins are induced by a wide range of metal anions and cations. Phytochelatin synthesis in suspension cultures was inhibited by buthionine sulfoximine, a specific inhibitor of gamma-glutamylcysteine synthetase (EC 6.3.2.2). This finding and kinetic studies of phytochelatin induction point to a synthesis from glutathione or its precursor, gamma-glutamylcysteine, in a sequential manner, thereby generating the set of homologous peptides.

Journal Article↗
PubMed1987

Analysis, cloning, and high-level expression of 2,4-dichlorophenoxyacetate monooxygenase gene tfdA of Alcaligenes eutrophus JMP134.

Plasmid pJP4 of Alcaligenes eutrophus JMP134 contains all genes for the degradation of 2,4-dichlorophenoxyacetic acid (2,4-D). Five of these genes, tfdB, tfdC, tfdD, tfdE, and tfdF, have recently been localized and cloned (R. H. Don, A. J. Weightman, H.-J. Knackmuss, and K. N. Timmis, J. Bacteriol. 161:85-90, 1985). Gene tfdA, which codes for the 2,4-D monooxygenase, has now been found by mutagenesis with transposon Tn5. A 3-kilobase fragment of pJP4 cloned in a broad-host-range vector could complement the 2,4-D-negative phenotype of two mutants which lacked 2,4-D monooxygenase activity. The cloned tfdA gene was also transferred to A. eutrophus JMP222, which is a cured derivative of JMP134. The recombinant strain could utilize phenoxyacetic acid as a sole source of carbon and energy. Pseudomonas sp. strain B13, containing the cloned tfdA, was able to degrade phenoxyacetic acid and 4-chlorophenoxyacetic acid. Gene tfdA was subcloned and analyzed by deletions. Expression of 2,4-D monooxygenase in Escherichia coli containing a 1.4-kilobase subfragment was demonstrated by radioisotopic enzyme assay, and a protein of 32,000-dalton molecular mass was detected by labeling experiments. A 2-kilobase subfragment containing tfdA has been sequenced. Sequence analysis revealed an open reading frame of 861 bases which was identified as the coding region of tfdA by insertion mutagenesis.

Alcaligenes↗
PubMed1979

Purification and properties of strictosidine synthase, the key enzyme in indole alkaloid formation.

A new enzyme, strictosidine synthase, which catalyzes the synthesis of 3-alpha(S)-strictosidine from tryptamine and secologanin was isolated from the soluble protein extract of Catharanthus roseus cell suspension cultures and was purified approximately 50-fold by ammonium sulfate fractionation, column chromatography on DEAE-cellulose. Ultrogel AcA34 and isoelectric focusing. The apparent molecular weight of the enzyme was 34000. The pH optimum was 6.8, apparent Km values for tryptamine and secologanin were 2.3 mM and 3.4 mM respectively for the enzyme to synthesize strictosidine. Strictosidine synthase shows high substrate specificity. No apparent cofactor requirement could be demonstrated. Of several enzyme inhibitors tested, only p-chloromercuribenzoate inhibited the enzyme. The enzyme was relatively stable and could be stored at -20 degrees C for periods of up to 1 year without appreciable loss of catalytic activity. The enzyme was demonstrated to occur in suspension cultures of 15 different species belonging to 9 different genera of the indole-alkaloid-producing subfamily Plumerioideae of the Apocynaceae family. This enzyme is responsible for the synthesis of strictosidine the key intermediate in the formation of the majority of monoterpenoid indole alkaloids occurring in the plant kingdom.

Cells, Cultured↗
PubMed1979

Iodinated digoxin derivatives with improved reactivity and stability, for use in radioimmunoassay.

We report the synthesis and properties of a new digoxin derivative, prepared by reacting cyanogen bromide-activated digoxin with tyramine. The derivative is extremely stable, and its monoiodinated form can be isolated in 85--90% purity by a single thin-layer chromatographic step. [125I]Tyramine-digoxin is immunologically stable for at least four months (immunoreactivities, 90% or greater throughout), with a shelf life of about six months. Nonspecific binding of less than 10% was observed for several batches during this time.

Digoxin↗