Intermediates in the recycling of 5-methylthioribose to methionine in fruits |
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Authors: | Kushad M M Richardson D G Ferro A J |
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Institution: | Department of Horticulture, Oregon State University, Corvallis, Oregon 97331-3804. |
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Abstract: | 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 α-keto-γ-methylthiobutyric acid (α-KMB) and α-hydroxy-γ-methylthiobutyric acid (α-HMB) by chromatography in several solvent systems. 35S]α-KMB was found to be further metabolized to methionine and α-HMB by these extracts, whereas α-HMB was not. However, α-HMB inhibited the conversion of α-KMB to methionine. Both U-14C]α-KMB and U-14C]methionine, but not U-14C]α-HMB, were converted to ethylene in tomato pericarp tissue. In addition, aminoethoxyvinylglycine inhibited the conversion of α-KMB to ethylene. These data suggest that the recycling pathway leading to ethylene is MTR → MTR-1-P → α-KMB → methionine → S-adenosylmethionine → 1-aminocyclopropane-1-carboxylic acid → ethylene. |
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