BIOSYNTHESIS OF PROTOHEME AND HEME-A FROM GLUTAMATE IN MAIZE
BIOSYNTHESIS OF PROTOHEME AND HEME-A FROM GLUTAMATE IN MAIZE
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DOI：
10.1104/pp.81.4.965
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发表时间：
1986-08-01
期刊：
影响因子：
7.4
通讯作者：
BEALE, SI
中科院分区：
文献类型：
--
作者：
SCHNEEGURT, MA;BEALE, SI
血红素和叶绿素前体δ-氨基乙酰丙酸（ALA）可以通过两种生物合成途径形成：通过叶绿体和蓝绿藻中的五碳途径从谷氨酸的完整碳骨架，以及通过细菌和动物线粒体中的ALA脱氢酶催化的琥珀酰辅酶A和甘氨酸的缩合。植物线粒体血红素a的生物合成途径通过在黑暗中用L-1-[14 C]谷氨酸（其仅可通过五碳途径掺入ALA）或2-[14 C]甘氨酸（其将通过ALA合酶掺入）孵育8天龄黄化玉米幼苗的末端上胚轴部分来确定。在高度纯化的血红素和血红素a中测量标记掺入。在12小时孵育中，标记物摄取大于70%。谷氨酸对总细胞原血红素的标记效率是甘氨酸的29.7倍。血红素A被标记的4.1倍，更有效地由谷氨酸比甘氨酸。为了评估两种氨基酸对血红素a的法尼基部分贡献标记的相对能力，测量了标记掺入总细胞非皂化脂质中。甘氨酸标记这一馏分11.3倍更有效地比谷氨酸。因此，甘氨酸对法尼基部分的贡献可以解释血红素a中出现的少量标记。我们的研究结果表明，在黄化的玉米，非共价结合血红素，包括线粒体血红素，大部分，并可能完全，从ALA通过五碳途径合成。通过ALA合成酶形成的ALA几乎没有贡献，并且没有发现这种酶在玉米中运作的证据。
The heme and chlorophyll precursor .delta.-aminolevulinic acid (ALA) can be formed by two biosynthetic routes: from the intact carbon skeleton of glutamate via a five-carbon pathway, which occurs in chloroplasts and bluegreen algae, and by ALA synthase-catalyzed condensation of succinyl-CoA and glycine, which occurs in bacteria and animal mitochondria. The biosynthetic route of plant mitochondrial heme a was determined by incubating terminal epicotyl sections of 8-day-old etiolated Zea mays seedlings in the dark with L-1-[14C]glutamate (which can be incorporated into ALA only via the five-carbon route) or 2-[14C]glycine (which would be incorporated via ALA synthase). Label incorporation was measured in highly purified protoheme and heme a. In 12-hour incubations, label uptake was greater than 70%. Total cellular protoheme was labeled 29.7 times more effectively by glutamate than glycine. Heme a was labeled 4.1 times more effectively by glutamate than by glycine. To assess the relative ability of the two amino acids to contribute label to the farnesyl moiety of heme a, label incorporation into total cellular nonsaponifiable lipids was measured. Glycine labeled this fraction 11.3 times more effectively than glutamate. Thus, a contribution by glycine to the farnesyl moiety may account for the small amount of label appearing in heme a. Our results indicate that in etiolated maize, noncovalently bound hemes, including mitochondrial heme a, are made mostly, and possibly entirely, from ALA synthesized via the five-carbon pathway. There is little or no contribution from ALA formed via ALA synthase, and no evidence was found for the operation of this enzyme in maize.