Two separate pathways for serine biosynthesis in animal systems have been established (9,18, 29, 31). The individual reactions in the pathway utilizing phosphorylated in,termediates (phosphorylated pathway) are given by reactions I to 3 and those involving nonphosphorylated compounds (nonphosphorylated pathway) are shown in reactions 4 and 5: 1) D-3-P-glycerate + DPN+ < > P-hydroxypyru- of the above palthw%rays for serine nmetabolism in plant systems has been reporte(l and is discussed below. Relatively few of these studies, however. have been carried out on the individual enzyi-Matic reactions in either of the 2 rouites. The phosphorylated pathway, described above in reactions 1 to 3, was fir-st demonistrated in highel plants by Hanfor(d and Davies (5) who showxed the over-all conversion of n-3-P-glycerate to i.-P-serine and L-serine in 1homogenates of pea epicotyls. In atddition, the results of i11 ivo() '4CO2 labeling experiments during short term photosynthesis are consistent with a functional phosphorvlate(d pathway for serine formation in many plant preparatiolls inclulding spinach chloroplasts (3), tobacco leaves (6), and algae (7). Furthermore, the occurrence in wheat germ of all of the enzymes of the phosphorvlated pathway, i.e. PGDH', P-hydroxypyruvate :L-glutamate transaminase, and phosphoserine phosphatase, has been demonstrated in this laboratory (unpublished observations).The second route for serine formation in plants is via the glycolate pathway, also known as the glyoxylate-serine pathway (16, 28
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