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Intermediates in the Recycling of 5-Methylthioribose to Methionine in Fruits ¹

Department of Horticulture, Oregon State University, Corvallis, Oregon 97331-3804, Department of Microbiology, Oregon State University, Corvallis, Oregon 97331-3804

The recycling of 5-methylthioribose (MTR) to methionine in avocado (Persea americana Mill, cv Hass) and tomato (Lycopersicum esculentum Mill, cv unknown) was examined. [¹⁴CH₃]MTR was not metabolized in cell free extract from avocado fruit. Either [¹⁴CH₃]MTR plus ATP or [¹⁴CH₃]5-methylthioribose-1-phosphate (MTR-1-P) alone, however, were metabolized to two new products by these extracts. MTR kinase activity has previously been detected in these fruit extracts. These data indicate that MTR must be converted to MTR-1-P by MTR kinase before further metabolism can occur. The products of MTR-1-P metabolism were tentatively identified as -keto--methylthiobutyric acid (-KMB) and -hydroxy--methylthiobutyric acid (-HMB) by chromatography in several solvent systems. [³⁵S]-KMB was found to be further metabolized to methionine and -HMB by these extracts, whereas -HMB was not. However, -HMB inhibited the conversion of -KMB to methionine. Both [U-¹⁴C]-KMB and [U-¹⁴C]methionine, but not [U-¹⁴C]-HMB, were converted to ethylene in tomato pericarp tissue. In addition, aminoethoxyvinylglycine inhibited the conversion of -KMB to ethylene. These data suggest that the recycling pathway leading to ethylene is MTR MTR-1-P -KMB methionine S-adenosylmethionine 1-aminocyclopropane-1-carboxylic acid ethylene.

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