Metabolism of sulphur amino acids via the cystathionine pathway in Argyrotaenia velutinana larvae

Abstract

In a previous study, cysteine and intermediates of the cystathionine pathway (homocysteine and cystathionine) were found to spare approximately 75 per cent of the dietary methionine requirement of Argyrotaenia velutinana larvae indicating a possible conversion of methionine to cysteine in this insect via the cystathionine pathway. To further verify the presence of the cystathionine pathway, studies of methionine and cysteine metabolism were conducted using A. velutinana larvae fed diets containing radioactive ( 35S) methionine or cysteine. Ion-exchange chromatography of the protein-free amino acid fraction of larvae fed diets containing 35S-methionine revealed the presence of radioactive methionine, methionine sulphoxide, cystathionine, and taurine. In the protein fraction radioactivity was found in methionine, methionine sulphoxide. and cyst(e)ine. Transfer of the radioactive label from 35S-methionine to cystathionine and taurine in the protein-free fraction and to cyst(e)ine in the protein fraction confirms the findings of nutritional studies that A. velutinana larvae are capable of converting methionine to cysteine via the cystathionine pathway. The absence of radioactive methionine, methionine sulphoxide, and cystathionine in the protein and protein-free fractions from insects fed 35S-cysteine showed that the cystathionine pathway from methionine to cysteine is not reversible in this insect. The presence of unknown radioactive peaks in the protein and protein-free fractions from larvae fed either 35S-methionine or cysteine indicated the possibility of sulphur amino acid metabolism by routes other than the cystathionine pathway.