Effects of Glycolate Pathway Intermediates on Glycine Decarboxylation and Serine Synthesis in Pea (Pisum sativum L.)

Abstract

ABSTRACIGlycine decarboxylation and serine synthesis were studied in pea (Pisum sativum L.) leaf discs, in metabolically active intact chloroplasts, and in mitochondria isolated both partially by differential centrifugation (i.e. 'crude') and by further purification on a Percoll gradient. Glycolate, glyoxylate, and formate reduced glycine decarboxylase activity ('4C02 and NH3 release) in the crude green-colored mitochondrial fractions, and in the leaf discs without markedly altering serine synthesis from 11-'4q glycine. Glycolate acted because it was converted to glyoxylate which behaves as a noncompetitive inhibitor (K, = 5.1 ± 0.5 millimolar) on the mitochondrial glycine decarboxylation reaction in both crude and Percollpurified mitochondria. In contrast, formate facilitates glycine to serine conversion by a route which does not involve glycine breakdown in the crude mitochondrial fraction and leaf discs. Formate does not alter the conversion of two molecules of glycine to one C02, one NH3, and one serine molecule in the Percoll-purified mitochondria. In chloroplasts which were unable to break glycine down to CO2 and NH3, serine was labeled equally from I'4Clformate and 1-'4Cjglycine. The maximum rate of serine synthesis observed in chloroplasts is similar to that in isolated metabolically active mitochondria. Formate does not appear to be able to substitute for the one-carbon unit produced during mitochondrial glycine breakdown but can facilitate serine synthesis from glycine in a chloroplast reaction which is probably a secondary one in vivo.

Effect of Oxygen Concentration on 14C-Photoassimilate Transport from Leaves of Salvia splendens L.

“…There is some debate whether photorespiratory serine synthesis requires NH3 release as several intercellular mechanisms (Fig. 4) may account for the synthesis of this amino acid (13,22). However, there is good evidence that most of the NH3 generated during glycine decarboxylation is refixed (12) and not released from the tissue (4).…”

“…4). The scheme in Figure 4 (6,7,22). Little photorespiration evidently occurs in sink leaves (20); therefore, photorespiratory activity in source leaves may provide key metabolites needed for development and maintenance of growing sinks.…”

The Intracellular Distribution of Folate Derivatives in Pea Leaves

“…Despite this, ihese organelles generale fomiylmetliionyl-tRNA (41), transmethylate homocysteine (42), convert formate to serine (43), and contain SHMT (44,45), 5,10-methylenetetrahydrofolate dehydrogenase (10) and FPGS activities (12). A more detailed assessment of the roles chloroplasts play in onecarbon metabolism appears warranted.…”

Impairment of Photorespiratory Carbon Flow into Rubber by the Inhibition of the Glycolate Pathway in Guayule (Parthenium argentatum Gray)

“…Leaf mitochondria were isolated and purified on a Percoll/ sucrose gradient (24). The activities of glycine decarboxylase were determined by the method of Shingles et al (24).…”

“…The activities of glycine decarboxylase were determined by the method of Shingles et al (24). SHMT was assayed according to Taylor and Weissbach (25) Protein content was estimated according to Lowry et al (17) using BSA as standard.…”