Properties and Intramitochondrial Localization of Serine Hydroxymethyltransferase in Leaves of Higher Plants

Abstract

The activity of serine hydroxymethyltransferase in mitochondria isolated from spinach leaves was absolutely dependent on tetrahydrofolate; pyridoxal phosphate has no effect on the activity. The stability of this activity in the isolated mitochondria was dependent on the presence of sulfhydryl compounds. It was apparently more stable at pH 7.0-7.5 than at higher pH, even though the pH optimum of serine hydroxymethyltransferase was 8.5 for both the mitochondrial and cytoplasmic fractions. Distribution studies have indicated that serine hydroxymethyltransferase was predominantly located in the mitochondria. The activity of serine hydroxymethyltransferase was co-compartmented with glycine decarboxylation and malate dehydrogenase behind the mitochondrial inner membrane. This activity could be solubilized by KCl from osmotically ruptured mitochondrial membrane fractions but substantial activity (35%-40%) was still retained with the membrane fractions at 0.3 M KCl. This suggests that the glycine decarboxylation-serine hydroxymethyltransferase complex may be closely bound to the internal surface of the mitochondrial inner membrane. The relationship of this integrated enzyme complex to CO2 evolution and serine synthesis during photorespiration and the physiological role of the dicarboxylate shuttle were discussed.