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The biosynthesis of defensive chemicals by millipedes : parrallelism with plant biosynthetic pathways Duffey, Sean Stephen

Abstract

The biogenesis of hydrogen cyanide. ( HCN ) and benzalde- hyde has been studied in two polydesmid millipedes, Harpaphe haydeniana and Oxidus gracilis. It was shown by feeding Oxidus gracilis D, L-phenylalanme- ¹⁴C labelled in the 1,2,3, or ring carbons that the ring and carbon-3 were utilized to make benz-aldehyde, and carbon-2 to make the cyanide carbon. It is postulated that this occurs by the catabolism of phenylalanine to mandelonitrile. It was found that Harpaphe haydeniana was also capable of incorporating D,L-phenylalanme- ¹⁴C specifically into HCN and benzaldehyde, as did O. gracilis. H. haydeniana was shown to store D-(R)-mandelonitrile as a droplet in the storage vestibule of each cyanogenic gland. Using ¹⁴C and ³H labelled compounds it was shown that H. haydeniana synthesizes HCN via a mechanism similar to that of plants. The pathway in the millipede appears to be: phenylalanine → N-hydroxyphenylalanine → ( phenylpyruvic acid oxime ?) → phenylacetaldoxime → phenylaceto- nitrile and/or 2-hydroxyphenylacetaldoxime → mandelonitrile. From feeding experiments using D,L-phenylalanme-ring- ¹⁴C,_D,L- phenylalanine-2- ¹⁴C and phenylacetonitrile-1- ¹⁴C it was possible to show the natural occurrence of four intermediates known to occur in plants; the oxime, the nitrile, mandelonitrile, and a glycoside of mandelonitrile. Two enzymes have also been isolated from H. haydeniana which are a part of this biosynthetic pathway; the β-glycosidase and the α-hydroxynitrile lyase. Both enzymes are associated with the cyanogenic gland. The pH optima for the enzymes are 4.2 and 6.0 respectively. The fate of HCN and benzaldehyde from decomposed mandelo- nitrile was determined in H. haydeniana. It was found that H ¹⁴CN was converted to β-cyanoalanme-¹⁴C and subsequently asparagine- ¹⁴C, as is known to occur in plants. Benzaldehyde -¹⁴C was converted to p-hydroxybenzoic acid-¹⁴C. The toxicity of HCN to two polydesmid millipedes, one parajulid millipede, one hemipteran, and one orthopteran was compared, and on a weight basis the millipedes were more resistant to HCN. The hypothesis for the mechanism of HCN production in polydesmid millipedes is appraised by new biochemical evidence. Employing biochemical and morphological data from this thesis and from the accepted hypothesis, a more detailed chemical and physical explanation is given for the production of HCN. The occurrence of HCN was demonstrated in six other polydesmid millipedes; Boraria stricta, Caraibodesmus sp., Pseudopolydesmus branneri, Polydesmus angustus, Nearctodesmus cerasinus, and Scytonotus insulanus. The nature of the ketone or aldehyde was only determined in H. haydeniana. It has been shown that Boraria stricta, Cherokia georgiana, Harpaphe haydeniana, Oxidus gracilis, Polydesmus angustus, Nearctodesmus cerasinus, and Scytonotus insulanus store a droplet of oil in each storage vestibule of the cyanogenic gland. This droplet was shown to be mandelonitrile in H. haydeniana and O. gracilis. Three p-benzoquinones have been identified in a Jamaican millipede, Rhinocricus holomelanus; benzoquinone, toluquinone, and 2-methyl-3-methoxy-benzoquinone. The biosynthesis of these chemicals appeared to be based on the catabolism of phenyl- alanine-¹⁴C and tyrosine-¹⁴C. The aromatic ring of these ammo acids gave rise to the nucleus of the quinone. The presence of these three benzoquinones has been detected in nine other millipedes of diverse families; Bollman-iulus sp., Eurhinocricus sp. nr. sabulosus, E. sp. nr. bruesi, Leptogoniulus naresi, Qrthoporus ornatus, Rhinocricus holo-melanus, Saiulus sp. nov. nr. setifer, Trigoniulus lumbricinus, and Tuniulus hewitti. No chemotaxonomic value can be ascribed to the occurrence of these benzoquinones in these species.

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