Zelitch I
Department of Biochemistry, The Connecticut Agricultural Experiment Station, New Haven, Connecticut 06504.
Plant Physiol. 1973 Feb;51(2):299-305. doi: 10.1104/pp.51.2.299.
After a preliminary period in light, leaf disks floated on 10 mm alpha-hydroxy-2-pyridinemethanesulfonic acid to inhibit glycolate oxidase accumulate glycolate at average initial rates of 67 micromoles in tobacco and 8 micromoles per gram fresh weight per hour in maize under optimal conditions in air. In the presence of (14)CO(2), the glycolate synthesized has a high specific radioactivity in illuminated tobacco and a low one in maize. Isonicotinic acid hydrazide also inhibits glycolate oxidation and causes a slow accumulation of glycolate in maize but not in tobacco, while it inhibits glycolate synthesis in tobacco but not in maize. Radioactive carbon in acetate-2-(14)C and especially pyruvate-3-(14)C is incorporated predominantly into the C-2 of glycolate in both species, but the specific radioactivity is much greater in maize. Glyoxylate-2-(14)C is readily converted to glycolate-2-(14)C in both species. The addition of phosphoenolpyruvate stimulated glycolate formation in maize and inhibited its synthesis in tobacco, and in the presence of (14)CO(2) the specific radioactivity in glycolate-(14)C was decreased greatly by the added phosphoenolpyruvate only in maize.Thus, unsymmetrically labeled glycolate is mainly synthesized from pyruvate-3-(14)C by a slow pathway in maize. Tobacco possesses an additional rapid pathway that produces equally labeled glycolate more directly from fixed CO(2) during photosynthesis. Glycolate is believed to be the primary substrate of photorespiration, and sufficiently rapid rates of glycolate synthesis have been observed in tobacco to account for this function. Hence the high rates of photorespiration observed in tobacco leaves compared with maize result partly from differences between these species in the pathway of glycolate synthesis.
在光照的初始阶段后，漂浮在10毫米α-羟基-2-吡啶甲磺酸上以抑制乙醇酸氧化酶的叶圆片，在空气中的最佳条件下，烟草中乙醇酸的积累平均初始速率为每小时67微摩尔，玉米中为每克鲜重每小时8微摩尔。在(14)CO(2)存在下，合成的乙醇酸在光照的烟草中具有高比放射性，而在玉米中则较低。异烟肼也抑制乙醇酸氧化，并导致玉米中乙醇酸缓慢积累，但在烟草中不积累，同时它抑制烟草中乙醇酸的合成，但不抑制玉米中的合成。醋酸-2-(14)C尤其是丙酮酸-3-(14)C中的放射性碳在两种植物中主要掺入乙醇酸的C-2位，但玉米中的比放射性要大得多。乙醛酸-2-(14)C在两种植物中都很容易转化为乙醇酸-2-(14)C。磷酸烯醇式丙酮酸的添加刺激了玉米中乙醇酸的形成，并抑制了其在烟草中的合成，并且在(14)CO(2)存在下，仅在玉米中添加的磷酸烯醇式丙酮酸使乙醇酸-(14)C中的比放射性大大降低。因此，不对称标记的乙醇酸在玉米中主要通过一条缓慢的途径由丙酮酸-3-(14)C合成。烟草具有另一条快速途径，在光合作用期间更直接地从固定的CO(2)产生等量标记的乙醇酸。乙醇酸被认为是光呼吸的主要底物，并且在烟草中观察到足够快的乙醇酸合成速率来解释这一功能。因此，与玉米相比，烟草叶片中观察到的高光呼吸速率部分是由于这些物种在乙醇酸合成途径上的差异。
