Kandra G, Severson R, Wagner G J
Agronomy Department, University of Kentucky, Lexington 40546-0091.
Eur J Biochem. 1990 Mar 10;188(2):385-91. doi: 10.1111/j.1432-1033.1990.tb15415.x.
A major diversion of carbon from branched-chain amino acid biosynthesis/catabolism to form acyl moieties of sucrose esters (6-O-acetyl-2,3,4-tri-O-acyl-alpha-D-glucopyranosyl-beta-D- fructofuranosides) was observed to be associated with specialized trichome head cells which secrete large amounts of sucrose esters. Surface chemistry and acetyl and acyl substituent groups of tobacco (T.I. 1068) sucrose esters were identified and quantified by gas chromatography/mass spectrometry. Sucrose esters were prominent surface constituents and 3-methylvaleric acid, 2- and 3-methylbutyric acid, and methylpropionic acid accounted for 60%, 25% and 9%, respectively, of total C3--C7 acyl substituents. Radiolabeled Thr, Ile, Val, Leu, pyruvate and Asp, metabolites of branched-chain amino acid pathways, were compared with radioactively labeled acetate and sucrose as donors of carbon to sucrose, acetyl and acyl components of sucrose esters using epidermal peels with undisturbed trichomes. Preparations of biosynthetically competent trichome heads (site of sucrose ester formation) were also examined. Results indicate that 3-methylvaleryl and 2-methylbutyryl groups are derived from the Thr pathway of branched-chain amino acid metabolism, 3-methylbutyryl and methylpropionyl groups are formed via the pyruvate pathway, and that acetyl groups are principally formed directly via acetyl-CoA. Arguments are presented which rule out participation of fatty acid synthase in the formation of prominent acyl acids. Results suggest that the shunting of carbon away from the biosynthesis of Val, Leu and Ile may be due to a low level of amino acid utilization in protein synthesis in specialized glandular head cells of trichomes. This would result in the availability of corresponding oxo acids for CoA activation and esterification to form sucrose esters. Preliminary evidence was found for the involvement of cycling reactions in oxo-acid-chain lengthening and for utilization of pyruvate-derived 2-oxobutyrate to form straight-chain acyl substituents.
观察到碳从支链氨基酸生物合成/分解代谢大量转向形成蔗糖酯（6-O-乙酰基-2,3,4-三-O-酰基-α-D-吡喃葡萄糖基-β-D-呋喃果糖苷）的酰基部分，这与分泌大量蔗糖酯的特化腺毛头部细胞有关。通过气相色谱/质谱法对烟草（T.I. 1068）蔗糖酯的表面化学以及乙酰基和酰基取代基进行了鉴定和定量。蔗糖酯是主要的表面成分，3-甲基戊酸、2-甲基丁酸和3-甲基丁酸以及甲基丙酸分别占总C3 - C7酰基取代基的60%、25%和9%。使用带有未受干扰腺毛的表皮，将支链氨基酸途径的放射性标记苏氨酸、异亮氨酸、缬氨酸、亮氨酸、丙酮酸和天冬氨酸代谢物与放射性标记的乙酸盐和蔗糖作为碳供体，用于蔗糖酯的蔗糖、乙酰基和酰基成分的研究。还对具有生物合成能力的腺毛头部（蔗糖酯形成部位）制剂进行了检查。结果表明，3-甲基戊酰基和2-甲基丁酰基基团源自支链氨基酸代谢的苏氨酸途径，3-甲基丁酰基和甲基丙酰基基团通过丙酮酸途径形成，并且乙酰基主要直接通过乙酰辅酶A形成。提出的论据排除了脂肪酸合酶参与形成主要酰基酸的可能性。结果表明，碳从缬氨酸、亮氨酸和异亮氨酸的生物合成中分流可能是由于腺毛特化腺头细胞中蛋白质合成中氨基酸利用率较低。这将导致相应的氧代酸可用于辅酶A活化和酯化以形成蔗糖酯。发现了初步证据表明循环反应参与氧代酸链延长以及利用丙酮酸衍生的2-氧代丁酸形成直链酰基取代基。
