Department of Botany, University of Western Australia, Nedlands, W. A. 6009, Australia.
Plant Physiol. 1983 Apr;71(4):841-8. doi: 10.1104/pp.71.4.841.
Net balances of amino acids were constructed for stages of development of a leaf of white lupin (Lupinus albus L.) using data on the N economy of the leaf, its exchanges of amino acids through xylem and phloem, and net changes in its soluble and protein-bound amino acids. Asparagine, aspartate, and gamma-aminobutyrate were delivered to the leaf in excess of amounts consumed in growth and/or phloem export. Glutamine was supplied in excess until full leaf expansion (20 days) but was later synthesized in large amounts in association with mobilization of N from the leaf. Net requirements for glutamate, threonine, serine, proline, glycine, alanine, valine, isoleucine, leucine, tyrosine, phenylalanine, histidine, lysine, and arginine were met mainly or entirely by synthesis within the leaf. Amides furnished the bulk of the N for amino acid synthesis, asparagine providing from 24 to 68%. In vitro activity of asparaginase (EC 3.5.1.1) exceeded that of asparagine:pyruvate aminotransferase (EC 2.6.1.14) during early leaf expansion, when in vivo estimates of asparagine metabolism were highest. Thereafter, aminotransferase activity greatly exceeded that of asparaginase. Rates of activity of one or both asparagine-utilizing enzymes exceeded estimated rates of asparagine catabolism throughout leaf development. In vitro activities of glutamine synthetase (EC 6.3.1.2) and glutamate synthase (EC 1.4.7.1) were consistently much higher than that of glutamate dehydrogenase (EC 1.4.1.3), and activities of the former two enzymes more than accounted for estimated rates of ammonia release in photorespiration and deamidation of asparagine.
利用叶片氮素代谢、通过木质部和韧皮部的氨基酸交换以及可溶性和蛋白结合氨基酸的净变化数据，构建了白 Lupinus albus L.）叶片发育阶段的氨基酸净平衡。天冬酰胺、天冬氨酸和γ-氨基丁酸的供应量超过了生长和/或韧皮部输出所消耗的量。谷氨酸在完全叶片展开（20 天）之前一直过量供应，但后来随着叶片中氮的动员，大量合成。谷氨酸、苏氨酸、丝氨酸、脯氨酸、甘氨酸、丙氨酸、缬氨酸、异亮氨酸、亮氨酸、酪氨酸、苯丙氨酸、组氨酸、赖氨酸和精氨酸的净需求主要或完全通过叶片内的合成来满足。酰胺为氨基酸合成提供了大部分氮，天冬酰胺提供了 24%至 68%。在早期叶片扩展期间，天冬酰胺酶（EC 3.5.1.1）的体外活性超过了天冬酰胺：丙酮酸转氨酶（EC 2.6.1.14），此时体内估计的天冬酰胺代谢最高。此后，转氨酶活性大大超过了天冬酰胺酶。在叶片发育过程中，一种或两种利用天冬酰胺的酶的活性始终超过估计的天冬酰胺分解代谢率。谷氨酰胺合成酶（EC 6.3.1.2）和谷氨酸合酶（EC 1.4.7.1）的体外活性始终远高于谷氨酸脱氢酶（EC 1.4.1.3），前两种酶的活性足以解释光呼吸和天冬酰胺脱酰胺过程中氨释放的估计速率。
