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

Abstract

Glycine decarboxylation and serine synthesis were studied in pea (P. 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 (14CO2 and NH3 release) in the crude green-colored mitochondrial fractions and in the leaf discs without markedly altering serine synthesis from [1-14C]glycine. Glycolate acted because it was converted to glyoxylate which behaves as a noncompetitive inhibitor (Ki = 5.1 .+-. 0.5 mM) on the mitochondrial glycine decarboxylation reaction in both crude and Percoll-purified mitochondria. 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 2 molecules of glycine to 1 CO2, 1 NH3 and 1 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 [14C]formate and [1-14C]glycine. 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 1 C 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.