Comparative Studies of Enzymes Related to Serine Metabolism in Higher Plants

Abstract

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P-hydroxypyruvate :L-glutamate transaminase, and phosphoserine phosphatase, has been demonstrated in this laboratory (unpublished observations).The second route for serine formation in plants is via the glycolate pathway, also known as the glyoxylate-serine pathway (16, 28

Development of Microbodies in Sunflower Cotyledons and Castor Bean Endosperm during Germination

“…20). Peroxisomal marker enzymes were found to be low, or even absent, in the dark, whereas light enormously increases their activity (5,8,11 15, 19, 23. 24, 28, 31,32,37,38).…”

Regulation of Serine Biosynthesis in Arabidopsis

“…The cDNAs encoding the four component enzymes of GDC (14 -17) and serine hydroxymethyltransferase (18,19) from plants have been cloned and characterized. The other two Ser biosynthetic pathways from 3-PGA via either phosphorylated or non-phosphorylated intermediates are proposed to be important Ser sources in non-photosynthetic tissues or under dark conditions, depending on the type of tissues involved (20) (Fig. 1).…”

Phosphoserine Aminotransferase in Soybean Root Nodules

“…The reaction mixture contained in a volume of 500 ul, 60 mM sodium borate (pH 8.2), 50 ,ug ml-' pyridoxal-P, 7.5 mM alanine, 4 Mig ml-' pyridoxal-P, 2.5 mM DTE, and 10 mg ml-' fatty acidfree BSA. The nodule extract (in 5 ml) was layered on a 30 ml linear gradient of 0.75 to 2.0 M sucrose in 20 mM tricine (pH 7.8), 2.5 mm DTE, 25 Mg ml-' pyridoxal phosphate over a 1.5 ml bottom layer of 2.3 M sucrose.…”

“…SPAT (EC 2.6.1.51) was assayed using a method based on that ofCheung et al (4). The reaction mixture contained in a volume of 500 ul, 60 mM sodium borate (pH 8.2), 50 ,ug ml-' pyridoxal-P, 7.5 mM alanine, 4 Mig ml-' pyridoxal-P, 2.5 mM DTE, and 10 mg ml-' fatty acidfree BSA.…”

“…SPAT is present in the leaves of higher plants (4) and so an extract of spinach leaves was assayed for this activity as a positive control to check the efficacy of the assay system. SPAT activity was detected in spinach leaves at levels comparable to those already reported (4). The data are consistent with there being either none, or extremely low levels, ofSPAT in the nodule and serine synthesis occurring via the phosphorylated pathway.…”