Institute of Plant Sciences, Newe Ya'ar Research Center, Agricultural Research Organization, PO Box 1021, Ramat Yishay 30095, Israel.
Plant J. 2013 May;74(3):458-72. doi: 10.1111/tpj.12149. Epub 2013 Mar 22.
Sulfur-containing aroma volatiles are important contributors to the distinctive aroma of melon and other fruits. Melon cultivars and accessions differ in the content of sulfur-containing and other volatiles. L-methionine has been postulated to serve as a precursor of these volatiles. Incubation of melon fruit cubes with ¹³C- and ²H-labeled L-methionine revealed two distinct catabolic routes into volatiles. One route apparently involves the action of an L-methionine aminotransferase and preserves the main carbon skeleton of L-methionine. The second route apparently involves the action of an L-methionine-γ-lyase activity, releasing methanethiol, a backbone for formation of thiol-derived aroma volatiles. Exogenous L-methionine also generated non-sulfur volatiles by further metabolism of α-ketobutyrate, a product of L-methionine-γ-lyase activity. α-Ketobutyrate was further metabolized into L-isoleucine and other important melon volatiles, including non-sulfur branched and straight-chain esters. Cell-free extracts derived from ripe melon fruit exhibited L-methionine-γ-lyase enzymatic activity. A melon gene (CmMGL) ectopically expressed in Escherichia coli, was shown to encode a protein possessing L-methionine-γ-lyase enzymatic activity. Expression of CmMGL was relatively low in early stages of melon fruit development, but increased in the flesh of ripe fruits, depending on the cultivar tested. Moreover, the levels of expression of CmMGL in recombinant inbred lines co-segregated with the levels of sulfur-containing aroma volatiles enriched with +1 m/z unit and postulated to be produced via this route. Our results indicate that L-methionine is a precursor of both sulfur and non-sulfur aroma volatiles in melon fruit.
含硫香气挥发物是瓜类和其他水果独特香气的重要贡献者。不同的瓜类品种和品系在含硫和其他挥发物的含量上存在差异。L-蛋氨酸被认为是这些挥发物的前体。用¹³C 和 ²H 标记的 L-蛋氨酸孵育瓜类果实小方块，揭示了两种进入挥发性物质的不同分解途径。一种途径显然涉及 L-蛋氨酸氨基转移酶的作用，并保留了 L-蛋氨酸的主要碳骨架。第二种途径显然涉及 L-蛋氨酸-γ-裂合酶活性，释放甲硫醇，这是形成硫衍生香气挥发物的骨干。外源性 L-蛋氨酸还通过 L-蛋氨酸-γ-裂合酶活性产物α-酮丁酸的进一步代谢生成非硫挥发物。α-酮丁酸进一步代谢为 L-异亮氨酸和其他重要的瓜类挥发物，包括非硫支链和直链酯。从成熟瓜类果实中提取的无细胞提取物表现出 L-蛋氨酸-γ-裂合酶的酶活性。在大肠杆菌中异位表达的瓜类基因（CmMGL）被证明编码一种具有 L-蛋氨酸-γ-裂合酶酶活性的蛋白质。CmMGL 在瓜类果实发育的早期表达水平相对较低，但在成熟果实的果肉中表达水平增加，这取决于所测试的品种。此外，CmMGL 在重组自交系中的表达水平与富含+1 m/z 单位的含硫香气挥发物水平以及通过该途径假设产生的挥发物水平共分离。我们的结果表明，L-蛋氨酸是瓜类果实中硫和非硫香气挥发物的共同前体。
