Herman W. Knoche

Structure of an amino acid analog of the host-specific toxin from helminthosporium carbonum

The structure of an analog to the host-specific plant toxin from Helminthosporium carbonum has been determined to be cyclo [prolylalanylglycyl-2-amino-8-oxo-9,10-epoxydecanyl], which has a glycine residue in place of the D-alanine residue of the toxin.

Sterols of Uromyces phaseoli uredospores

Abstract The major sterol of bean rust uredospores (Uromyces phaseoli) has been identified as 7,(Z)-24(28)-stigmastadien-3β-ol. A second sterol component appears to be 7-stigmasten-3β-ol. Both steroids are synthesized by the organism during the germination of the uredospores.

Essentiality of the ketone function for toxicity of the host-selective toxin produced by Helminthosporium carbonum☆

Abstract The cyclic tetrapeptide, cyclo-(-Pro-Ala-Ala-Aoe), which is a host-selective toxin (HC-toxin) produced by the maize pathogen, Helminthosporium carbonum race 1, was reduced with sodium borohydride. Reduction converted the 2-amino-8-oxo-9,10-epoxydecanoic acid (Aoe) residue to a 2-amino-8-hydroxy-9,10-epoxydecanoic acid (Ahe) residue. Two isomers were isolated and shown by NMR to be diastereomers of cyclo-(-Pro-Ala-Ala-Ahe) that differed by their configurations of carbon atom number eight of the Ahe residues. Neither isomer, alone nor as a 1:1 mixture, was toxic to lines of maize sensitive to HC-toxin. Consequently the ketone group of the Aoe residue in HC-toxin appears to be necessary for the toxicity of this host-selective toxin.

Origin of sterols in uredospores of Uromyces phaseoli

Abstract The sterols from healthy bean leaves are β-sitosterol, stigmasterol, campesterol and 28-isofucosterol. An additional sterol observed in bean leaves infected with Uromyces phaseoli was identified as 7,(Z)-24(28)-stigmastadien-3β-ol, which is the major sterol of the uredospores of the fungus. The fungus appears to stimulate sterol synthesis, but most of the increased sterol content of infected leaves can be attributed to the sterol of the uredospores.

Sterol methyltransferase from Uromyces phaseoli: An investigation of the first and the second transmethylation reactions☆

The two-carbon unit at C-24 of many plant, algal and fungal sterols is known to be synthesized by two successive transmethylations with S-adenosylm

The backbone and side chain conformations of the cyclic tetrapeptide HC-toxin.

A study of the conformational parameters of HC-toxin and its diacetyl derivative in chloroform solution has been carried out. Two-dimensional NMR spectroscopy and the nuclear Overhauser effect have been used in order to determine connectivities (assignments and sequence) and approximate torsion angles and interproton distances. The results are consistent with a bis-gamma-turn conformation previously reported for dihydrochlamydocin. Model building based upon NMR data supports a D configuration for Ala2 and Pro4 residues.

A sterol methyltransferase from bean rust uredospores, Uromyces phaseoli

Abstract A methyltransferase(s) that catalyzes the transfer of the methyl group from S-adenosylmethionine to a sterol acceptor was solubilized with Triton X-100 and partially purified from bean rust uredospores ( Uromyces phaseoli ). Zymosterol was the most active substrate tested while desmosterol and lanosterol exhibited good activity. The products were sterols with either a methylene or ethylidene group at the C-24 position. Direct evidence for the synthesis of the ethylidene group was obtained by using 24-methylenecholesterol as a substrate.

The effect of volume and quenching on estimation of counting efficiencies in liquid scintillation counting

Abstract The effect of volume on the liquid scintillation counting performance of 14C-samples has been investigated. A decrease in counting efficiency was observed for samples with volumes below about 6 ml and those above about 18 ml when unquenched samples were assayed. Two quench-correction methods, sample channels ratio and external standard channels ratio, and three different liquid scintillation counters, were used in an investigation to determine the magnitude of the error in predicting counting efficiencies when small volume samples (2 ml) with different levels of quenching were assayed. The 2 ml samples exhibited slightly greater standard deviations of the difference between predicted and determined counting efficiencies than did 15 ml samples. Nevertheless, the magnitude of the errors indicate that if the sample channels ratio method of quench correction is employed, 2 ml samples may be counted in conventional counting vials with little loss in counting precision.