Bruin W J, Nelson E B, Tolbert N E
Department of Biochemistry, Michigan State University, East Lansing, Michigan 48823.
Plant Physiol. 1970 Sep;46(3):386-91. doi: 10.1104/pp.46.3.386.
By three criteria, the glycolate pathway of metabolism is present in unicellular green algae. Exogenous glycolate-1-(14)C was assimilated and metabolized to glycine-1-(14)C and serine-1-(14)C. During photosynthetic (14)CO(2) fixation the distributions of (14)C in glycolate and glycine were similar enough to suggest a product-precursor relationship. Five enzymes associated with the glycolate pathway were present in algae grown on air. These were P-glycolate phosphatase, glycolate dehydrogenase (glycolate:dichloroindophenol oxidoreductase), l-glutamate:glyoxylate aminotransferase, serine hydroxymethylase, and glycerate dehydrogenase. Properties of glycerate dehydrogenase and the aminotransferase were similar to those from leaf peroxisomes. The specific activity of glycolate dehydrogenase and serine hydroxymethylase in algae was 1/5 to 1/10 that of the other enzymes, and both these enzymes appear ratelimiting for the glycolate pathway.Labeling patterns for products of the glycolate pathway during (14)CO(2) fixation are not the same as those obtained with higher plants. In higher plants glycolate, glycine, and serine are uniformly labeled at shortest time periods. In algae, serine was predominately carboxyl-labeled, similarly to 3-phosphoglycerate. This result, plus the lower specific activity of serine hydroxymethylase, indicates that the glycine-serine interconversin in algae is slower than in plants. Initially (2 to 4 seconds) glycolate and glycine were more C-2 labeled. They rapidly became uniformly labeled, with glycine becoming uniformly labeled first. In the presence of isonicotinylhydrazide, labeled glycolate and glycine accumulated, and only a trace of serine-(14)C was detected. Then glycolate and glycine were initially carboxyl-labeled, and glycolate became uniformly labeled almost immediately and before glycine. These results suggest rapid metabolism of glycolate and glycine, in addition to the glycolate pathway.
根据三个标准，单细胞绿藻中存在乙醇酸代谢途径。外源1-(14)C乙醇酸被吸收并代谢为1-(14)C甘氨酸和1-(14)C丝氨酸。在光合(14)CO₂固定过程中，乙醇酸和甘氨酸中(14)C的分布足够相似，表明存在产物-前体关系。在空气中生长的藻类中存在与乙醇酸途径相关的五种酶。它们是磷酸乙醇酸磷酸酶、乙醇酸脱氢酶（乙醇酸：二氯靛酚氧化还原酶）、L-谷氨酸：乙醛酸氨基转移酶、丝氨酸羟甲基化酶和甘油酸脱氢酶。甘油酸脱氢酶和氨基转移酶的性质与叶片过氧化物酶体中的相似。藻类中乙醇酸脱氢酶和丝氨酸羟甲基化酶的比活性是其他酶的1/5至1/10，这两种酶似乎是乙醇酸途径的限速酶。(14)CO₂固定过程中乙醇酸途径产物的标记模式与高等植物不同。在高等植物中，乙醇酸、甘氨酸和丝氨酸在最短时间内被均匀标记。在藻类中，丝氨酸主要在羧基处被标记，类似于3-磷酸甘油酸。这一结果，加上丝氨酸羟甲基化酶较低的比活性，表明藻类中甘氨酸-丝氨酸的相互转化比植物中慢。最初（2至4秒），乙醇酸和甘氨酸的C-2标记更多。它们迅速变得均匀标记，甘氨酸首先变得均匀标记。在异烟酰肼存在的情况下，标记的乙醇酸和甘氨酸积累，仅检测到微量的1-(14)C丝氨酸。然后乙醇酸和甘氨酸最初在羧基处被标记，乙醇酸几乎立即且在甘氨酸之前变得均匀标记。这些结果表明，除了乙醇酸途径外，乙醇酸和甘氨酸还存在快速代谢。
