Carrier Protein-mediated Transport of Neutral Amino Acids into Mung Bean Mitochondria

Abstract

The transport of neutral amino acids into mitochondria isolated from the hypocotyl of mung bean (Roxb.) was studied by the swelling technique. Isolated mitochondria sweled when added to an isosmotic solution of proline, serine, methionine, threonine, alanine, and glycine. The swelling was stereospecific in that it was faster in the L-amino acid than in the corresponding D-amino acid. Preincubation of the mitochondria with the sulfhydryl modifying reagents,p-mercuribenzoate and mersalyl, resulted in an inhibition of the swelling caused by proline, serine, threonine, and glycine. The sweling induced by alanine was inhibited only by mersalyl, whereas that by methionine was inhibited only byp-mercuribenzoate. In all cases, the inhibition caused by the sulfhydryl modifying reagents was readily reversible by the subsequent treatment of the mitochondria with dithiothreitol. N-Ethylmaleimide, another sulfhydryl-modifying reagent, did not cause any inhibition of the swelling. The findings indicate the existence of a protein mediated mechanism for the transport of neutral amino acids into plant mitochondria.In plant cells, numerous metabolic events involving the reactions of neutral amino acids occur inside the mitochondria. The interconversion of serine to glycine and a one-carbon fragment in the mitochondria is well-documented (6,16,20). Since the mitochondria contain several aminotransferases specific for some neutral amino acids (25), the interconversion of these amino acids with their corresponding keto acids should occur inside the organelles. In addition, there are two major metabolic events involving mitochondrial metabolism of neutral amino acids that are unique to higher plants. One of them is the operation of the glycolate pathway in leaves during photorespiration (16,24). In this pathway, glycine formed in the leaf peroxisomes enters the mitochondria and is oxidized to form C02, NH3, and a one-carbon fragment which will join with another glycine to form a serine; serine subsequently leaves the mitochondria to be converted to sugar eventually. The other metabolic event is proline metabolism in connection with the accumulation of proline under environmental stresses (2,4,14,23). Although the detailed mechanism of proline metabolism is still unclear, it is known that the enzymes of proline catabolism are localized in the mitochondria, whereas the anabolic enzymes are present in the cytosol (1, 2,15,23

Oxidation of Proline by Mitochondria Isolated from Water-Stressed Maize Shoots

“…Ultrastructural examinations ofwater-stressed tissue have found physical damage to mitochondrial membranes of maize roots held over saline solutions causing a severe 75% fresh weight loss (19 [5] used 200 mm proline for their swelling-transport experiments) such as that required for glutamate entry into cauliflower bud and beet root mitochondria (7). Glutamate transport is inhibited by 2-n-butylmalonate and is suggestive of a dicarboxylic anion/phosphate exchange, unlike the neutral characteristics of proline in our experimental conditions.…”

Plant mitochondrial carriers: an overview

“…Further experiments need to be performed in order to clarify the features of these carriers. The importance of the malate-oxaloacetate shuttle in the transfer of reducing equivalents has been demonstrated [201][202][203][204][205]. During photorespiration, the reduction of hydroxypyruvate in the peroxisomes requires reducing equivalents, which are produced either during the photosynthetic electron transport in the chloroplast or during substrate respiration in the mitochondria.…”

Mitochondrial Cysteine Synthase Complex Regulates O-Acetylserine Biosynthesis in Plants

“…In agreement with this proposal, the NMR studies of isolated mitochondria provide direct evidence for the export of OAS from mitochondria. Neutral amino acids have been shown to permeate across the mitochondrial membranes by carrierand/or channel-mediated processes (30,31). To date, however, the identities of the mitochondrial OAS exporter and the transporters for serine and most other amino acids remain unknown in plants (32).…”